"What makes the Beretta so heavy?"
It's simply a function of what's known as the size-to-caliber (STC) ratio, a problem found in overly large handguns chambered for diminutive calibers.
In the Beretta's case, say the model 92, there's way more metal, and therefore more weight and size, than there needs to be given the puny caliber for which it's chambered (.355/9mm).
This results in a structurally-inefficient pistol, one that's needlessly large and heavy for the tiny 9mm cartridge. And although it's possible to keep a straight face when telling the uninitiated that your Beretta is a "manly handcannon," those who've done their STC homework know otherwise.
Note that mag capacity is not a relevant justification for a sloppy STC ratio. The Glock 19, an example of a structurally-
efficient 9mm pistol having a proper STC ratio, has a 15+1 capacity (or, with a +2 extention, 17+1).