Nope I was wrong it doesn't let you do that, I haven't played FSN in ages. Sorry about that, my bad.
Eitherway, this pretty much is most peoples argument;
In order to compensate for this disadvantage, he modified his body. As a result, he was able to rival other Servants.
It's basically saying that he modified his body to be able to compete with other servants.
However your assassin probably has a skill, history, or NP that lets them be that stronk, or I assume so. I'll place faith in that for now and believe you and our GM. It isn't impossible for there to be a strong assassin, you just need a good justification, and you obliviously can't show that right now for identity hiding reasons.