Erm what?If it is smooth at higher revs, it can still be low compression, years ago I had a Ford V6 that I hammered down the motorway and the next day it wouldn't tickover, I had broken the compression rings on two cylinders. It turned out the engine had not been driven hard before and there was a wear ridge on the top of the cylinder bore where the pistons usually stopped before going back down, my higher revving made the pistons go a fraction higher causing the piston rings to touch the ridge. In those days I was able to borrow a "ridge cutter" to grind away the ridge. It was also possible to buy ridge dodger piston rings that had a step to miss the ridge. Nowadays the proper course would be a rebore and a set of new pistons.
You may be able to detect a weak compression the way I suggested. Another sign of low compression on a diesel is poor starting from cold accompanied with white smoke, although usually once running for a bit they tickover OK.
If you have tried everything else a proper compression test would eliminate that.
The travel of the position is set by the length of the rods and Pistons
It should have no difference on if it's at 50 rpm or 2000 how could the piston travel any further up the bore at higher rpm