Colossus isn't really painful for the legs... more on the head. For safety reasons guests must be properly secured in the seat, which may not be possible for some guests with larger torso or thigh measurements. If you are unsure, selected rides have test seats that may be used before entering the queue line. As Pluk said,
I wouldn't recommend it either and also I wouldnt recommend Submission at Alton Towers as IT HURTS LIKE HELL! I'm very skinny and that is coming from a 12 year old
Apart from that I hope you have a brilliant holiday and that you dont scream to much