video - ffmpeg transcode twice compress further?

I have a collection of avi videos (say A).

I first convert them into MPEG4 Part2 (say B) using ffmpeg -i ${inname} -vf scale=340:256,setsar=1:1 -q:v 1 -c:v mpeg4 -g 12 -f rawvideo ${outname}. The total file size is 3.5G.

Then using B as input, I apply the same command to transcode B again into C, which has size 3.1G.

I thought B and C have the same format and thus should have the same file size? Why not and how can I resolve it to make sure no information loss in the second transcoding process?