Remember that it is easier to fit "x" number of .45 rounds into a polymer pistol than into a metal-with-grip panels pistol, if you keep the outside dimensions the same. So the Glock 30 is smaller but holds 10 also.
Apples, oranges, you know the saying.
I am not too keen on the aluminum frame SIG in .45. With the slightly higher bore, it recoils up too much for my taste. I haven't had the fun of firing the all steel SIG 220. I'd probably like it quite a bit.
“There’s a rattle in my glove box,
A Federal twelve-gauge slug.” – Adam Carroll in song Porter Wagoner