Let's take a peek at what Glock says about the weight of it's guns.
Glock claims the G19 loaded weighs 30.16 ounces. That may vary a bit depending on round count. The pencil barreled 4" S&W weighs 29.5 ounces and will vary some depending on stocks.
So the loaded G19, to no ones surprise, weighs either a bit more or within a couple of ounces of the M&P. Loaded though and the steel M&P goes over the weight of the G19 by a bit. The stocks, Pachmyers rubber grips for example, may increase that as well.
My mistake and the mistake of two other shooters at a local gunatorium recently was based on picking up both guns empty side by side and holding them and subjectively weighing them. We all agreed and were surprised that the M10 felt lighter than the G19. Still feels
lighter but that is just balance. I don't think this point is particular to Glocks. Some of the polymer framed guns, depending on barrel length, have a certain muzzle heavy feel to them when unloaded. The balance point of the gun will shift when a full mag is inserted. But a bull barrel on the M10 and it will feel a good bit heavier than the G19.
I don't recall why I made this point to begin with.