I have to tell you that I have no clue at all because I am new to chickens as well, but I would almost swear that bigger chick is older than the other one. My chicks (including three Buff Orps that are supposed to be pullets) will be 4 weeks old tomorrow or maybe Saturday (they arrived in the mail on a Monday morning) and I have been taking pictures nearly everyday since then. The giant chick looks like my chicks at about three-four weeks.
Also, I have been posting about my straight-run Marans for help on figuring out the sexes and I've had several people tell me that the boys usually feather out more slowly than the girls. Then again, I, too, have a giant chick (mine is a Marans) that is also way more feathered out than the rest, and from all the pics, everyone agrees he's a boy. So, I have to say, I'm not so sure anyone can really, truly tell until they start crowing or laying...
Here are some pics for reference:
at two weeks:
at three weeks:
3.5 weeks:
I have no idea if any of that helps because I am still figuring things out, but it just truck me when I saw your pictures that the giant chick just seems older in every way... which could mean boy for all I know. Oh yeah and all my Buff Orps had a little early comb growth...