Actually the .50 isn't designed to turn small props fast. It is designed to turn larger props faster than the smaller displacement engines can.
The .50 being a bored out .45 tends to be flow restricted due to the shallower inlet ports, so it doesn't breathe as well at higher rpms.
But it'll turn a larger prop better than the smaller engines can. The additional displacement helps there.
But as I remember it right, the .50 could turn a 10x6 at about 500 rpms more than a 45 could.
Where it winds up being good is like for a large 40 size scale plane where you want some extra oomph. Or a 40 size 3D plane where you want more vertical performance out of it.
The .50 is able to turn a 11x7 prop pretty well. The 11x7 prop used to be what we used on all the big .60's years ago.
The .40's back then used 10x6 props or maybe a 11x5 or 11x6 sometimes.