PDF only with JPG photo is 7 times larger than original

Yo do not have a JPG file anymore.

When you embed a photo inside a document, you do not have a JPG file anymore, you have a document with a photo inside. And this image is now subject to the new file's compression rules.

The abrupt change in the file size of the PDF is most likely due to two things:

You do not have an RGB file anymore. You are converting it to CMYK which has, not only more channels but compresses less efficient than an RGB file. You could even use some JPG compression but this compresses every channel separately, versus the way a standard RGB file is compressed, where a luminescence channel and two chroma channels are used.

You are not using a JPG compression, but a lossless ZIP one. But even this one would only apply on similar grounds if you maintain an RGB file.

Keep in mind that you are NOT saving a PDF, you are exporting a file to PDF. The difference is that a saving tries to keep the data as it is as possible, including some effects that are reloaded and re-rendered, while an export makes a series of processes, like converting files, resampling, merging layers, antialiasing, converting vector effects to raster, among other things, depending on the target format and the configuration.