Trouble generating correct srcset sizes using getSrcset()

I’m trying to figure out the getSrcset() method, going by the docs here.

When I try it using the code in that example (I modified to account for a Matrix field but other than that it is the code below), the results aren’t what I’d expect. It takes the asset and INCREASES the sizes; what I would expect is the uploaded asset is considered the maximum resolution, and it is DECREASED from there.

Here is the code from that example page:

{% do asset.setTransform({ width: 300, height: 300 }) %}
{{ tag('img', {
    src: asset.url,
    width: asset.width,
    height: asset.height,
    srcset: asset.getSrcset(['1.5x', '2x', '3x']),
    alt: asset.title,
}) }}

Basically I can’t figure out how to tell it to scale down, not up.
I’m also not clear on what that first line is doing (width: 300, height:300) because it doesn’t set my images to those sizes.

Edit
Just to clarify, I know you can pass in actual sizes instead of x-descriptors, but what I can’t figure out is how you can use the x-descriptors without ruining the image by scaling it up, which defeats the whole point of serving high-res images.

Update
I ended up just using Image Transforms, but I’d still be curious to hear if I was using the getSrcset() wrong and if there is a way to scale down instead of up.