I am struggling with tiny diagrams in some epub and mobi files. Can anyone suggest Mac software which lets users (a) find all the images, and (b) enlarge them?

I have tried the Calibre reader. It doesn't help me find the images, and tends to yield blurry results if I zoom them.
Those blurry results are telling you that the images are low resolution. Zooming them does no real good unless you enjoy pixelation.
Use the Calibre editor to enlarge the images as much as you can before they become too pixelated/blurry. That's about the best you can do.

Those must be rather old eBooks to make such small images.
A while back, I looked at the ebook version of a high school level math book. The images didn't look bad at the normal resolution but when you zoomed them, they looked like crud. Evidently, nobody thought that anyone would zoom the images to be able to actually read the equations. I used the epub3 version and generated new images from the MathML and used those to rebuild the ebook (MathML => SVG => PNG). Looked much better but the ebook size went from 2.1MB to 24MB even with lossless compression applied to the mass of images.
Keep the SVG images as they don't get pixalated or blurry if you zoom in.
I didn't need the SVG versions with epub3 since MathML seems to work with kepub but the original owner was using a Kindle.
AH. WE just went through this--teeny-weeny equations, in Wordperfect, heavens help us, meant for ePUB and MOBI. For MOBI, we still use jpg/png, as the fallbacks need to work. We can't only use SVG and not all the software readers support SVG, either.

Thousands of the damn things. We spent a ton of time making them large enough to zoom properly and small enough to not look ridiculous inline. What a hassle. SVG would be soooooooooooooooo much easier...

Hitch