compact is about 2mm shorter from bottom to top. same barrel, same slide, slightly curved mag base for extra finger placement. Otherwise identical.
I have just tried out a compact "M" which has the single stack mag for reduced cross section. I've waited nearly 6 months to try and find a Beretta 92 series which my wife could operate size wise due to basic understanding of the M9 version. For myself anyway I was disappointed, The full size for me is a perfect size fit for my hand and the M version had terrible ergonomics for me. I may just wait to see if the 9000 fits both of us better, and obtain a full size border marshal with the barrel kit for myself.
