The difference is in the detail. Literally, the amount of detail. Diglett has a simple design but detailed artwork, just look at the soil around it.
Drillbur has more intricate design, but its eyes and mouth look pasted on, nose painted on, claws look like paper, toes aren't defined, etc.
Diglett is supposed to be a whack-a-mole, so the simple shape makes sense. It doesn't make sense on Drillbur, who's supposed to be an actual animal.
