How have you specified the MP3 compression? If it's bits per second, a
mono version *will* be identical in size to the stereo version.

Also, I believe MP3 uses a mid/side representation, since (as in FM
stereo) you can assume that most of the information is actually in the
mid so side can be compressed more agressively. If so, a mono signal
encoded as a stereo MP3 is all mid, no side -- and the genuinely mono
MP3, again, would be essentially the same data.

So that's two possible explanations for your observation.