Sensor size and pixels amount impact on image quality

Image quality is highly correlated with pixel-size. A larger pixel accumulates more light during an exposure of a certain duration. By having more light, there is less noise in the image because the pixel gets more signal from light and noise is relatively stable. So proportionate, which is what we perceive, images are less noisy.

Larger pixels also have a larger well which is the capacitor that accumulates the electrical charge that photons release when they hit the sensor surface. These deeper wells allow a pixel to represent more dynamic-range. It is applicable to scenes of high contrast, where smaller pixel reaches its maximum level faster and clips image highlights.

Now, technology has created a lot of optimizations for it such as BSI (Back-Side-Illumination) that allows silicon of the same size and number of pixels to effectively have larger pixels because more photons landing on the sensor get used, so pixel size alone is not an absolute measure, but it is far more accurate than resolution.

When comparing two cameras, what you can look at is the pixel-pitch which some sites list in specifications or you can estimate it yourself. Taking a full-frame sensor of 36x24mm and divide it by resolution (Say 26,000,000 for the 26 MP 6D Mark II) and compare it to 22x15 for Canon APS-C (or 24x16 for APS-C of other brands) and divide by the resolution (Say 32,000,000 for the Canon 90D). It is not necessary to use exact numbers because there are a lot of factors at play (which is why a 24 MP APS-C camera today gets better output the a 24 MP of 10 years ago) but this pixel-size calculation gives you good idea of relative quality.

Now if you were to take an image from a certain sensor and scale it down, you are effectively simulating larger pixels, there is more noise usually than if the sensor had a lower native resolution. So, if you want to compare at equal print sizes, you can divide by the resolution you intend to use for a given print size.

Camera makers know this which is why some - and Fujifilm had a particularly clever way of doing this - bin pixels to simulate larger pixels. They essentially group a number of pixels together, typically 4 (in a 2x2 grid), and output one image pixel for each 4 sensor pixels. This improves image quality at the expense of resolution.

Addendum: As someone pointed out. Lens has a great impact on image-quality. This comparison is purely from a sensor design but if a lens cannot deliver sufficient resolution, then output quality gets reduced but it does not affect noise or dynamic-range. Again there are subtitles since some lenses can have such strong vignetting that they effectively diminish light which makes noise apparent in peripheral areas.

Pretty much every aspect of image quality is determined by the amount of light received/recorded for the image. And it is primarily a light/area consideration; larger sensors have a larger area, and receive/record more light for any given composition.

The only aspect pixel size really relates to is resolution; how much you can enlarge/magnify that light/area and how much detail is recorded. There are two sides to this; if the lens cannot deliver the resolution then the sensor cannot record it. A larger sensor is also capable of recording more lens resolution, and conversely it is less demanding of lens resolution. With modern sensors it is most common for image IQ to degrade due to a lack of light, or a lack of lens resolution (technique), well before sensor resolution becomes the limit... If you are not looking at a pixelated image; then the individual sensor pixel considerations are not the limiting factor.

I would generally put IQ considerations in order of:

1. Technique
2. Lens
3. Sensor Size/Design
4. Sensor Resolution

But there are times where 2-3 might be reversed... what actually matters most to you may change for a given situation. I would suggest you don't get too concerned about it; the difference between APS and FF are not huge, and to actually see the benefits of the larger sensor is expensive (see 1-2 above).

I am not comparing the two models you are quoting. I will only make some general observations.

We need to define what do we mean by quality.

1. Noise

A more modern sensor will generally better low light performance. Noise is more noticeable in darker zones.

A bigger sensor is better for reducing the noise because it has more information from more photons "to average" on each sensor element (sensel).

2. Sharpness

Images produced by a lens has a limit of resolution. Imagine that one X lens projects a thin line but a bit blurry because it reached the limit of the lens. Let's say that this line falls on one line of big sensels, but on smaller sensels, it occupies one and a half sensel. The line will be a bit blurrier on the smaller sensor. Smaller sensor+more megapixels = smaller sensel.

3. Bokeh

Using one f 1.8 lens on a bigger frame will give you more bokeh than the same FOV equivalent lens on a crop frame.

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On these three categories, a bigger sensor will produce overall a better quality image.

It is not that image quality does not improve using more Megapixels, but I think the improvement of Mpx is not linear, but a curve, where you benefit more when the pixel count starts, but at some point, you need to, let's say, double the pixel count to notice an improvement. But only before reaching the capabilities of the lens.

4. Some other considerations

Dynamic range, processing of the image on the camera´s chip, color accuracy, etc.

Both sensor size and the number/size of the photosites¹ that sensor contains can have an effect on the images taken using that sensor.

You seem to be wondering if your 32MP APS-C camera is "better" than a 26MP FF camera. The answer is that neither is inherently better than the other for all conceivable use cases.

Which is "better" for taking a specific photograph depends entirely upon many other variables you haven't identified.

• What is the lens resolution? Is it higher or lower than the sensor(s) being considered?
• What is the dynamic range of the scene being recorded? Are there very bright and very dark parts in the scene, or is most of the scene closer to the same brightness level?
• What are the applicable limits to exposure time and aperture that may limit the total amount of light collected? Is the light too dim to use base ISO and the largest aperture available and still use an exposure time short enough to freeze subjects in motion?
• How large and heavy can the camera and lens be before it begins to affect one's ability to get the shot at all? Is the subject a 20 mile hike from the nearest road?

If you're working in low light and need to limit exposure time to prevent motion blur, then larger sensels are usually more important.

If you're taking tripod mounted landscape photos using multiple exposures to capture the entire dynamic range of the scene and want to create very large viewing sizes, then more pixels are probably more important to you.

If you've got a near unlimited lens budget so that the higher cost of longer focal length lenses with faster apertures is not a factor, then a larger sensor with larger pixels is probably your best choice. But if you've got a tight lens budget and desire to get as much "reach" out of the lenses you can afford, then higher pixel density on a smaller (and cheaper) sensor may be the better choice, even for the same use case of photographing birds in flight.

There are many other conceivable use cases in which a larger sensor with larger photosites may be more beneficial. There are also many use cases in which a smaller sensor with more dense resolution can work to one's advantage.

Cameras are tools. Just like many other kinds of tools, which one is "best" depends upon the job one desires to do using them.

A 24 oz. framing hammer would be practically useless for driving railroad spikes into crossties. An 8-pound sledge hammer is better for that. On the other hand, an 8 pound sledge hammer would be practically useless for framing a house. That's what the framing hammer was created to do well. In order to pick the right tool for the right job, one must first know what the job is.

_{¹ Technically, sensors don't have "pixels", they have photosites a/k/a sensels a/k/a pixel wells. A pixel is a discrete unit of a digital image when it is stored or displayed.}