Its not that simple really, basically any player can play any position no matter what size (well, with limitations of course

). Just because someone is 125kg doesn't mean he will be destined to be a prop forward, a good example of this is Jonah Lomu. 120kg-ish but could still run 100m in 11 seconds. I'm 70 kg/156lbs and only 170cm/5'7 tall but I play in the back row in U17s, and i'd like to say I play well there. Most people say I should play wing because i'm on the short side, but, I personally don't think I could play in the backline (I used to play halfback, but was dead boring and I wasn't that great), I like to think I play well in the forward pack. You could be 98kg and play in the backline, meanwhile another 98kg player could play in the second row even though both the same weight, its all down to technique really. Only you could choose the position you play, so you have to be honest in yourself and decide what you're good at, and what your not.
Like are you quick, or a good defender, have a good kicking game, light on your feet (agility) etc., then use your strengths and weaknesses into deciding a good position for yourself.