[AlanHK]

The size of the file depends mostly on the images.

When a file is converted to Mobi/AZW, all images become JPEG or GIF.

GIF images pass as-is, JPEGs may pass, or get compressed more.

All other formats will be converted to one or the other. PNG often ends up as a much larger, and lower resolution, JPEG, some small ones might become (larger) GIFs.
So I never use PNG in ebooks.

If you have line work, like a cartoon, black and white, you can make a very compact and very sharp image as a GIF that will pass to the Kindle.

I use Photoshop to sharpen, convert to greyscale, then save as PNG.
Then I use Irfanview to reduce the bit depth.
4 bits if it's just lines, 6, 8 or 16 if there are greys.
I convert and if noticeably degraded I undo and try with more colours.
Then I look at the palette and adjust the "black" to be actually black.
Then save as GIF.

The size of the image in pixels is even more important. If you can reduce that without noticeable degradation, then try that. I'm happy with 800 px wide. The PW is 1072 px wide, so no point going higher than that, unless you think people will be looking at it in the app on iPads or the like.