The OP asked:
So is the whole point then of having a mid-size is having more rounds? I mean I know its better to have it and not need it than not have it and need it.
No, higher capacity is not the whole point, though it is a potential advantage.
Larger handguns are generally easier to shoot than are smaller handguns, due to several factors:
1 - Longer sight radius is typically easier to aim;
2 - Greater mass makes recoil control and follow up shots easier;
3 - Larger grip options may provide a better grip to hand fit for some of us;
4 - Better fitting grip (leverage) plus more mass (inertia) typically means the gun is easier to keep steady through the trigger pull.
So, higher ammo capacity is certainly not the only reason to carry a larger handgun. I am carrying a 1911 commander type right now, which "only" holds 8+1 - but I shoot that gun very well.
Still, I really see no advantage to downloading a handgun below its design specs.