Actually, only his height is a 20th of his original size. Thanks to the square-cube law, his weight is much, much less.
@torchmlp was nice enough to do the math out for us, and they were curious about cursed Jacob’s strength compared to his weight, so they did the formulas out.
We know now that Jacob weighs about as much as three crayons, and 40x his weight is equal to the vase he was lifting. Dean, of course, can lift this vase in one hand and trap Jacob in a fist without the kid’s knack doing him a bit of good.
If we took cursed Sam, cursed Dean and cursed Jacob, the difference in their strength would be that together, Sam and Dean at a regular height would be able to pick up the Impala and walk away with it, while Jacob by himself could do the same with even less effort. Just one kid. Walking away with that car (put it back!).