Convert from epbup to mobi or awz3 - huge size and other questions

Hi all
I install calibre to ubuntu .
I have epub file and test convert it to mobi and azw3. On setting i change this
Disable font size rescalle - checked
Subset all embeded fonts - checked

results:
original epub - 3.8Mb
mobi - 4.1Mb
aw3 - 4.6Mb

Is this normal?

Who to use - mobi or azw3?
Advantages of mobi is less size.
Please tell me are there any reasons to use azw3 ?