Less velocity creates less expansion. Less expansion creates less frontal area which creates less drag which results in more penetration. Look at the pictures of bullet expansion at different speeds on the new Nosler bullet boxes. Easy to visualize the different penetration.
Slow lead bullets the buffalo hunters used would penetrate several feet in a buffalo. At the Battle of Adobe Walls, indians were killed by Sharps bullets that were fired through huge stacks of buffalo hides many feet thick that the attacking indians were using for cover.
I once had a 150 gr deer bullet from a .308 Win blow up completely on a deer's rib at less than ten yards. My .38 pistol would have been more effective on that shot at that range. The same deer bullet would probably have 'over penetrated' the same deer at 500 yards. A .22 LR bullet that doesn't deform will penetrate over 12 inches of lengthwise jackrabbit. In general, higher velocity means less penetration. To expect a bullet to expand the same at differing velocities is not usually going to happen.
