Gotta say, I recently picked up a HK45c and am enamored with it, so grain of salt.
But I looked at the full size too, and it does feel a bit bigger, IMO than the FNX, though I always have felt the FNX feels a little flimsy especially with no mag.
IMO I like the 45c better. The full mag with the "horse hoof" floor plate feels really good in the hand and it shoots better than my full size 1911s, at least in terms of recoil and getting back on target quick and easy.
Mine carries as easily as my Glock 19 (spare mag does take more work to conceal) and shoots nearly as well in rapid shooting drills, I can still manange a sub 5 second FAST consistently, which I can't do with my VBob or Glock 30S (when I had it). Not quite to the 4.5 or under I do with the 19, but I've only been at it a few weeks.
My HK45c is a V1 so it's got the safety/decocker and I really like it, hate it on the P30 as the shorter grip puts the decocker in an awkward and uncomfortable place for my 1911 trained hands.
My only complaint with the HK45c vs the full size is the grip is a bit slick. I'm no HK purist but I dont actually like the P30/HK45 grip all that much so I don't miss that, I just wish it had a bit more texture. Tried the rubber Talon grips and they didnt quite do it, probably just do some skateboard tape on the front/backstraps next.
Extra mags are really hard to find right now, if that's a consideration.