I don't know how it went in the comics, but in the 2nd big hulk movie (pardon the pun) that used that weedy guy instead of reusing the guy who then became nero in the jj-trek film, and liv tyler for the llittle miss bi.. er... you know, generals daughter girl.
anyway in that one, a soldier takes waay too much of the hulkserum thingy, and becomes that beige-skinned mega monster.
Well, what if, two plot bunnies::
Xander is the son of a hulk-type person, and his abilities are activated a certain night... well, any particularly emotional one (Night buffy died or when Angelus went to kill her only to be stopped by Xander, which given what canon made him like for most of the time and only slightly the rest of it, that seriously doesn't add up even with what was apparently said. Not seen the ep, but fanfics quite often have lots of scenes that are otherwise word for word to the scenes in the show. whats a copy gonna do to angelus? )
the bunny I thought of. Soldierguy was that guy from the film/comic version/whatever, only Xander got a tiny bit of it, so strength and drop in IQ a bit, but otherwise still xander as xander shaped and body builder version in hulk form. Green skin optional.