Quick size up...
Kahr PM9 comes in smaller in height, length and width (at the belly) and weighs in at 1 pound 3.8 ounces loaded.
S&W 642 is thinner in the barrel, more rounded and weighs in at 1 pound .8 ounces loaded.
Keep in mind, the PM9 also holds 2 more rounds with the 6 round magazine, which comes to .9 ounces with the ammo I use.
I was really surprised at how little the weight difference came out to be since everyone raves about the J-Frame being a great pocket gun, but almost everyone says the PM9 is too big and heavy.
Maybe it's the smoother curves and narrow barrel of the 642 that makes it different, me personally - I can't tell much difference, however initial thoughts are that the 642 seems to be a bit more comfortable IWB and in my Smartcarry.