I don't have any ammo here at work, but one internet source lists .44 ounces for a 124 gr 9 mm round. I'll check when I get home and update this post if I find that's wildly wrong.
So, 16 rounds in the Glock would weigh just over 7 ounces. Add that to the 21-ounce unloaded weight, and you get 28 ounces, which is still less than the unloaded Model 10.
I agree that the weight distribution and density makes a difference in how heavy a gun "feels", though. Back when I had my Glock 19, it never felt *that* much lighter than my pre-Model 10.