In case you are wondering if you can actually "feel" the difference ... I say Yes.
In comparison to the M&P-9, the G19 definitely feels lighter. And due to the lighter weight, the G19's recoil feels faster. There is more snap to it. The G19 is quick to return to target though.
Size wise, the G19 is about 1/2" ~ 3/4" shorter in both length and height. Width is the same. For me, I feel that the G19 is the perfect size and weight (well, it could be a thinner, like my 3914). Ergonomics suck though, which is why I use the M&P.