A Merlin engine has maximum revs of less than 3000 rpm, so a large prop with lots of pitch and blade area is required in a full size Mustang.

A model engine (even a big one in a 40 size model like a YS110) revs at over 9000 rpm, so the prop is smaller.

There are lots of scale effects like this, mainly because if a linear dimension, say , doubles, the weight increases 8 fold.

-David C.