Scale matters. For example, at large scale you might be able to essentially ignore the friction between two materials, but at 1/1000 scale it might be the same order of magnitude as the forces expected during operation.

While a large piece of sheet metal might slide down a given ramp, a 1/1000 scale bit of foil of that same metal would probably just sit on that same ramp and not move.

There are lots of constants that, by definition, don't scale.