Something may be off, but let me ask you something.
Can these guys tap you out reliably? In my opinion any normal-sized purple belt should be able to find an answer to a strong 250 lb-er with no grappling training. Are they able to demonstrate their jiu jitsu on you in live rolling, where you’re actively trying to stop them?
If so, I would give the guys the benefit of the doubt to be honest. I’d also advise you not to get too hung up on body type. There’s no 250 lb guy cheat codes you’re really missing out on at this stage. I’m sure the best heavyweight instructors could give you all kinds of adjustments that might help, but you still have to make the triangle. It’s still the same basic motion, and just doing your best for now will get you plenty far. Drill it as best you can. Take your time and do it as close to right as you can, and over time you’ll find yourself improving.
The triangle is a motion you’re training for a very specific situation, just like everything else in jiu jitsu. You’ll be better able to adjust for your body type after you understand the basic concept of a triangle and it’s application in the jiu jitsu landscape that becomes increasingly broader for you as you continue to train.
You’ll figure out what works best for your body type in different situations by learning the entire suite of techniques being taught and then trying to apply them in live training. There is no secret sauce for big guys, you have to walk the same path everyone else does in training.