[MA] Sweeps and SM question

[callen]

I've been trying to understand how Size Modifier and Sweeps work. I thought I understood the mechanics, but most the GURPS rules seem to work off reality pretty well and if I understand these right they're way off. So I figure I don't understand them. I couldn't find the answer when searching other threads, either.

On your chance to hit, you get the normal bonus for being smaller or penalty for being larger. This sets your effective skill level for the attack. That should be what you use for the contested roll to knock your opponent down. This is where I had thought I understood things.

But now let's have a mouse try to sweep a human. Let's say the mouse is an aggressive little critter (but still a mouse, albeit a clever one to consider a Sweep) and has a little bit of brawl skill, and let's assume a normal person not heavily trained in martial arts or anything like that. There is a huge size differential. The mouse will almost always hit and will be rolling the contested roll against a number in the high teens, near 20. Meanwhile the human rolls the contested roll against a number closer to 10. It seems as though the human who fails to Dodge the mouse's Sweep will almost always fall over. What??? I could stand still (not Dodge) and never fear a mouse knocking me over with a Sweep, no matter how aggressive the mouse. And I'm no combatant. (Well, I trained in biathlon and fencing, but not this type of fighting.)

It seems to me SM should provide a bonus in the reverse direction for the contested roll if the Sweep hits. But I cannot find anything outside of the Grappling rules to hit that suggests that, and I don't see that the Grappling rules to hit apply to the contested roll for Sweep.

Thanks,
Chris