Compressing a selection of 9 language lesson files (total 176MB FLAC) with no filters results in the following total size and bitrate range (low-median-high).

Q1 28.5MB 46-52-83kbps
Q2 27.9MB 46-51-80kbps
Q3 34.6MB 57-64-96kbps
Q4 42.9MB 71-79-134kbps
Q5 65.9MB 111-122-207kbps

I want a conversion product I can trust; this doesn't seem like the best start. It seems unlikely to expect a convincing answer that doesn't involve a bug, but you can try.

-Mr. Audio