My P-07 feels heavy because there are 15+1 147gr Federal HST's in it. Unloaded it doesn't feel too heavy to me. I recently picked up a titanium alloy guide rod off of eBay, which while heavier than a stock polymer guide rod, is significantly lighter than the stainless steel one I had been carrying in it. I've thought about carrying lighter bullets to knock some ounces off of the loaded weight of the gun, but haven't convinced myself it's a good idea yet.
Why is it heavier than a Glock 19? More steel parts inside. See all above answers... mainspring, hammer, hammer strut, etc. plus I think the slide is a little beefier.