i know rider weight has a significant advantage over hp and acceleration.
just look at pedrosa (5'2" 105lbs), he's been pulling higher top speeds on the RC211V compared to hayden(152 lbs). of course, hayden is winning more bec he's just the better rider right now. pedrosa is 2nd though, in the pts standing.
anyway... back to topic.
would it be a disadvantage for a lighter rider to force the bike into corners since they have less mass to push the bike?
There are no disadvantages that I know of being a lighter rider. Lower weight is actually easier on the tires in cornering. Holding a line is better. More weight wants to force you into a straight line. Less weight in bike and rider ='s nimble.
It depends on your suspension setup more than anything else. A properly adjusted suspension accounts for rider weight and mitigates the effect as much as possible.
The really cool thing about being light is it allows you to race in smaller engine classes. A heavy guy is at a serious disadvantage running 250s and 125s... when you move up to bigger engines the rider weight advantage becomes less relevant. At 1000CCs it really doesn't matter whether the rider weighs 130 or 230.