At 5'8, 180 pounds, I'm not that big, but I have quite a lot of muscle on me, and a built body plus wrestling experience. I was able to just outmuscle and beat a lot of the beginning BJJ guys when I first started out in the art too. But the higher belts would just kick my butt with gi chokes, etc.
I found out that gi chokes actually work well on big guys with big necks. I was able to tap out a 250 pound judoka on several occasions with the same cross choke from guard.
Keep working your technique, fake them out with some things and try to sweep or catch them with a surprise armlock, etc.
But you know the old axiom, The bigger they are!.......the more bones they break
Ryu