I am trying to convert my flac files to apple's AAC format.

I am using ANYMP4 converter and dbPoweramp converters. The first generates raw AAC files; the latter generates m4a files. What surpised me most is the size difference on the output files (for the same quality: 256kbps).

Why is dbPoweramp bumpting up the file size (twice that of the outputs of the ANYMP4 converter)?