I think their fights are balanced because he's faster than she is and he's probably better at reading her attacks and adjusting since he's a very skilled fighter (shown by how he approaches fighting Illyria).

But I think Spike's speed makes up for the deficit in Buffy's strength. Momentum gives power to punches. So while he can't benchpress as much as she can, he can probably come close to hitting her with a similar amount of force because of speed.

(I've talked about this too much with my PhD in Physics dad...and asked him to write meta on Slayer strength)