iPod Podcast conversions -- what's going on?

[Mustafa]

If I download a 90 min podcast via iPodder X, the mp3 file in iTunes is 16.8 MB. If I choose the 'convert to AAC and make bookmarkable' option, the file becomes a 42.3 MB m4a.

First question -- why does it become so large -- wouldn't you expect it to be smaller than the mp3?

Second question -- why doesn't it become an m4b file -- the audiobook format that would make it bookmarkable?

 

[Cybernanga]

Hve you checked that you are not re-encoding at a higher sample rate. That would be the first thing I would check if the m4a is larger than the mp3