if you consider his ARMOR part of the storyline... that's weird.
Yes it is weird, but i have a habit of nitpicking at things
And with that said, i'll continue nitpicking by saying;
but it really doesn't matter and would be worse if he looked the same from Halo 1 all the way through.
He wouldn't be wearing the same armor from Halo, he'd be wearing the same armor from Halo 2, there wasn't much of a change from 2 to 3, as i've said, only the gloves, not that big of a deal, but from 3 to 4, he has a sort of thrusters on those upper back parts that bulge out or something and i don't know if this image shows how his armor is staying or if it'll change, but it's a HUGE difference from Halo 3

It barely looks like the Mark VI armor anymore, it looks like a different armor type, not saying it's bad, it just doesn't look like the Mark VI armor anymore, though the helmet has a sort of resemblance still, can't say much for the lower portion of the body, cause i haven't seen a good quality image of that