MK Kahrs are smaller than the P-11, but only a little bit. Kahrs are definately slimmer than the P-11, but heavier too. The Kahr has lower magazine capacity.
Some have had problems with the P series Kahrs, others have not. I have no experience with P series Kahrs, but I love my MK40.
IMO the Kahr MK9 is the smallest reliable 9mm out there. The MK40 is slightly larger than the MK9, but tiny for the punch it packs! The MK40 is less than 1" thick, about 5.5" long and just over 4" high.