I wondered the same thing, but I get same (+/- 100 bytes) results as the OP, the output AZW3 is a bit bigger than the input EPUB.
Not in the GUI, I removed the cover, the feedbooks logos and <img> tags from body using Sigil, saved and then converted that at command line to AZW3. That would have used the default EPUB input and AZW3 output settings. Maybe PeterT has tweaked the conversion settings.
The 3 images were ~830K bytes.
BR