some (many) bearings are specced as being ~1/3 to ~1/2 grease /total housing volume, e.g.http://connect.timken.com/packing-tapered-roller-bearing-with-grease
There are two reasons for this in (relatively, vs bicycle hubs) high speed applications;
1) the excess grease comes out.
2) the bearing can overheat.
In a car wheel bearing both are possible, and when the grease comes out, it can find its way onto the brake discs with predictable consequences. Roller bearings work the grease harder than ball bearings, and generate more heat when overpacked by some margin. For really high speed running, wheel bearings should be oil lubricated. Famously this was proven (the hard way) when Art Arfons' wheel bearing on his 'Green Monster' land speed record car failed at ~600mph. The bearing overheated, seized, then sheared the stub axle off, all in about three seconds. It could be that a witness to this prang originated the 'rolled up into a little ball' description of the results. Mr Arfons was lucky to survive.
Overpacking a bicycle wheel bearing may result in a little extra friction, maybe some lubricant loss, but isn't likely to be harmful otherwise in the same way; speeds are just not high enough for that. Obviously using a thixotropic semi-fluid grease gives oil-like levels of friction and seal lip wetting in use, but without the same risk of static leakage past less-than-perfect seals when the bike is stored.