AVI is a 'container' just like MOV on the mac... which means, that various codecs can be applied to the video within it, so.. just because it's an AVI doesn't mean it's BIGGER...
You could do an Xvid, or Divx encode and save as an avi.. and it would be smaller..
AVI stands for 'Audio Video Interleave' .. it is the standard format for a PC.
By the way,
This is the direct link for FlaskMPEG.. and I believe it is only available for the PC.. but there may well be a similar Mac program..