Well, they could always pull a Captain America and make him scrawny. Though from what I understand, there's no origin, so he'll be spiderman from the start, in which case him being buff-er makes a little more sense.
As for the real reason, sex sells and marvel has been very transparent with their BUFF NO SHIRT policy for male characters, so there's always that.
Way to treat a non-profit labor of love. What have you ever done?