Why screen size varies across games?

[KThunder]

Amiga applications can also be coded to use overscan. This increases the resolution slightly and allows them to display full screen.

standard 320x200 uses a border around the displayed area. The overscan settings put that higher at something like 356x220 putting the area all the way to the edges to the screen.

There are different settings for some apps like dpaint with normal, overscan and maximum overscan. Games can do this too.

[KThunder]

so you are using a pal amiga, what are the games? are they pal or ntsc? I think overscan is more likely a reason for the varying size of screens. Overscan uses more memory so many games dont use it.

I have only ever had ntsc amigas and games and the screens on many were different.