Well, its not merely the size... its the fact that a ball is 3d SPHERE... and the 2d game did
not consider that factor.
A spheres contact on the backboard is merely a small point in the center. Once you
go past that point, the surface of the ball is then contacting at a different surface point,
and the ball easily falls into the nearby hole.
Think bowling ball on a ledge. Once its past the center point (more than half way over
the ledge) , it will no longer be balanced, and will roll off the ledge.
Maybe Future Pinball supports full 3d sphere detection? If not, then maybe you might
look into something like Blitz Basic 3D.
Some interesting Facts:
1) On the real game... a big problem is Dirt/Dust. A small flake can cause the ball to
get hung on on the leveling bar. It can also slow down the ball travel (easier game)
and or cause erratic unwanted movement.
Being that the game has a huge complex pathway system in the rear, a Lot of crud
accumulates in there. The only way to get the game working well, is to tear the entire
game down and clean every surface. The rear assembly needs about 50 screws
undone to get it off!

2) The game is stupid hard... UNTIL you figure out the trick. Which is basically to
use both sticks at the same time. IE: Push one stick up, while pushing the other
stick down. (helps to keep the bar from tilting too much) Or pressing both sticks up
or down at the same time... making a perfectly level travel.
The game then become almost too easy. Its still a challenge.. especially when you
get higher up... or are impatiently trying to rush, to get a good time/score.
3) Another issue with them tends to be the Bar assembly itself. The joints tend to get
worn and loose.. and do not function properly anymore. I had to rebuild mine. I luckily
found the proper sized tubing that worked out perfectly.
4) The belts are often destroyed due to old age. Cracked/brittle and they slip. They
can be replaced with windsheild washer hose from an automobile store. I used
cableties to mend the ends together.
5) Being mechanical, the micro-leaf limit switches can be hard to keep in-check.
Needed adjustment every so often.