Hmmm, I'd imagine they are about the same.supersquad3 said:
In 26x2.1 I have the UST crossmarks ~730g
In 26x2.25 I have the standard RR's ~515g
I'd say the two you mentioned (29x2.1 vs 29x2.25) would be within 50g / tire. But this is conjecture only.
Schwalbe list accurate weights on their site. So just throw your crossmark on a scale and you will have your answer.