Is 10mm big?
The short version is the 10mm has a longer case, but the. 45 ACP has a larger projectile. The 10mm also has a shorter bullet.

Is 10mm Bigger Than 45?

10mm Auto .45 ACP
Bullet diameter 0.4 in 0.451 in
Case diameter (neck) 0.425 in 0.473 in
Case length 0.992 in 0.898 in
Overall length 1.26 in 1.275 in