He's got a bit of growing to do yet, Radar was much smaller at 6 months than he is now at 9 months, he's had a real growth spurt over the last couple of months and hasn't finished growing yet (he's cornish rex/siamese/dsh cross). Generally speaking, cats with oriental/refined body types mature a bit younger than cobby/stocky types, and females are smaller than males (in general).
But a lot of it comes down to the individual, if he is healthy and active I wouldn't worry about him being smaller than his brother, even littermates can end up different sizes. I asked the vet on our last visit with Radar about whether his weight was OK (I'm a worrywort) and she was happy to pop him on the scales and confirm that he was a good weight (ie neither under nor over for his body type). She reckoned that he would reach his full size and weight by 13-15 months, being a shorthair cornish rex/siamese cross.