I'm sure that's exactly the problem.
A 1911 is a large frame handgun. A G19 is not.
And the MARSOC .45 loaded is closer to 44oz.
Not to mention just about any equivalent load out you care to build will be approximately double the weight for the 1911 config.
It's not like a pistol is the only thing these guys have to carry. Ounces equal pounds, and pounds equal pain.
“A 1911 is a large frame handgun”
No, it is not. It is one of the slimmest pistols there is. You have obviously never held one.