The whole "Smaller dogs live longer than big dogs" thing due to breed or actual dog size?

Our Labradoodle puppy is getting up there in age (she's 12 :3), and it's got me thinking about how much longer she's gonna be around. I've always heard that larger dogs usually die naturally younger than smaller ones and was wondering if it's because of the genes of larger dog breeds or due to the animal itself being physically larger. Ours was the runt of the litter and is noticeable much shorter than her siblings, them being normal Labrador/ medium Poodle height and her being like at their shoulder. She's pretty much 100% healthy (has a few fat lumps we've had checked and occasionally forgets she's ancient and hurts her leg from overdoing it😅) otherwise, so we're hoping we've got years left instead of like months.