Offline bouncing a stereo file in Pro Tools HD 12.4 and 12.5 (not tested with earlier versions) results reproducibly the size of bounced left channel to be same than the source channel of the bounce, but right bounced channel is a bit larger (reported by Finder). Same happens with 5.1 files - the size of bounced left channel is the same than the original, all the other five are a bit larger.
Pro Tools sees the bounced files as same size anyway, importing them automatically into multichannel track. This does not happen in Pro Tools 11 HD. I have repeated this on several different systems, so it's not system specific.