M4A files (from YAMB) = slightly smaller filesize than original AACs?

[Christopher Woods (CustomMade)]

I've been taking some previously downloaded raw AAC files (downloaded with
get_iplayer) and wrapping them as M4As using YAMB. For e.g.:

Randomfile.m4a = 111 MB (116,501,077 bytes)
Randomfile.aac = 111 MB (117,371,538 bytes)

I've observed that consistently the resulting M4As are slightly smaller than
the raw AACs. Is this to be expected? Is it something to do with the way the
stream is muxed? I'm certainly not intentionally altering any aspect of the
file during the M4A creation, so how come the M4As (which one would
logically expect to be slightly larger given the wrapper overhead) end up
being the opposite?


Also is there anything technically incorrect about creating audio-only MP4
files as opposed to M4A? (I've noticed that YAMB is intelligent enough to
'do things' appropriate to creating M4A files if you specify the output file
extension to be M4A instead of the default MP4, I'm not a big user of either
MP4 or AAC though outside of the scope of archiving my get_iplayer downloads
- predominantly x264 in MKVs for me).

TIA (and yes, I did UTFG ;-)
Chris