BJJ grappling question?

I am 150 lbs and I have so much trouble with guys who are about 200 lbs and over. We can be in the same belt rank but they just are way too strong. Is this normal? I just want to make sure that it really is a huge difference. It's killing me because guys in my rank aren't so difficult and I can beat them most of the time. These guys that way at least 50 lbs heavier than I are just incredible.