Man I have the same problem. A couple guys at my gym are 260 plus with good BJJ and a little bit of height on me (I'm 6ft 145). My best solutions I have come up with so far is to play a little more open and wild then I like to. Personally I have a hard top getting top position on them in a scramble so more often than not I pull guard and try to throw up pop triangles or quick armbars (the armbars are much lower % for me). Also keep an eye out for inverted triangles especially the one setup from a recent MTS I have been having some success with that on the bigger guys as well. Like someone else said leg locks are always an option, I have been working on adding Einsteins ankle lock to my game but I haven't caught it on any of the big guys yet (havent went for it on them either though.) Other than that the other thing I look for on them is free New Yorks from the cocoon or any position where you see the opportunity, I have trouble getting the normal step by step rubber guard on the really big guys but if I can get free New Yorks I at least have something to work with. Good luck hope this helps....try to stay away from
half guard with way heavier guys too, I get smashed a lot if I try to play half with them. Like others have mentioned arm drag to the back and prison guard are great options too...in fact i think im going to make prison guard my new project.