By the way, if I choose to convert a file to AZW3, does it mean I get KF8? (And the file would be smaller than what I get right now)
Yes. For all practical purposes, an AZW3 is a stand-alone KF8 file... and it will definitely be smaller than the combined mobi/KF8 file (using the same input file, of course). Just be aware that not all devices/apps will be able to read
that AZW3 file.