this argument has been argued over since the birth of trials, the key is to get the balance right. if you are a heavy rider due to over eating and you use weights to get fit, you will just get heaveyer because muscle weighs more then fat, and if you choose uber light parts the strength will decrease in the parts and so the heaveyer rider will have more chance to break them. So basically in an ideal world you should pump some iron and get a strong steal bike