I have both the full size M&P 45 and the M&P 45 compact. As far a size relation, the full size is nearly identical to a Glock 17 or Glock 22 (though the slide is a hair longer on the M&P). The compact is the same roughly as the Glock 19 or Glock 23 and in my opinion is the perfect size for every day concealed carry.

I do not think the full size is too big for concealed carry, but that being said, the full size is my night stand gun and the compact is my off duty carry gun. I carry both IWB, but have largely relegated the full size to the night stand with X300 attached and 14 round mags with 10-8 baseplates.

The full size has a 4.5" barrel and the compact has a 4" barrel. The compact does not have a beavertail, and for me is more comfortable to carry. Weight wise they are only about 1.5 ounces appart, so for me it is a size thing and not a weight thing. My advice would be to keep your full size and also get a compact so you can have a CC and HD weapon that operate the same, and take each others magazines.