you better be joking...
If you originally had a medium and it was tight...That's good. Having a lose helmet is actually highly dangerous. f you fell and hi your head you could get the result of a Double concusion because your head has the room to move around...If it was a little tight your head wouldn't budge and there would be less consequence to the fall. Because being a Big racer myself, Falling and crashing is a expectation in motocross. Good Luck!
P.S: Whoever else answered this question half assed and gave him shitty answers, you arn't ment to ride Motocross...I'm a girl and I am close to pro...So suck that. good Luck Buddy.