Why can't people under 110 lb. just give a smaller amount of blood?

I always feel left out at blood drives. I’m young, reasonably healthy, can regenerate my blood as quick as the next non-anemic individual, and do not in any other way fall outside the eligibility guidelines, but I’ve never weighed 110 pounds and for that they refuse me. I understand it’s a matter of keeping me from passing out from blood loss, but why couldn’t they just take 10% less than they usually do? My first thought was that it’s not worth the equipment and manpower, but they always seem to need blood pretty badly, so my confusion persists. Thoughts?