Didn't want to say it before...but, basically yeah - there you go. Not something that can be said out loud, so glad someone else did.
Am nowhere near super heavyweight, but in most European countries if you're 80kg or above you're basically considered a giant monster, and 2/3 of the room simply don't want to partner up with you. So you get friendly with that final 1/3. When the others do spar with you it's because the trainer tells them to, and then they have this energy of annoyance and bitchy suppressed anger - you can feel it, just seeping out of their pores. And yes, most of them do exactly what you just described, start unloading on you at 100%. As if they just watched you run over their dog in your driveway.
The reason? Again, one of those things that you're simply not allowed to talk about, but some people really really believe this. That they're allowed to hit someone harder than they're being hit. Genuinely. This guy's got 5 more fights than me? I'm allowed to hit harder. He's an inch taller? I'm allowed to hit harder. He's heavier? I'm allowed to hit harder. He had 3 karate lessons when he was 12 years old? I'm allowed to hit harder. They don't accept that productive sparring has an unspoken, unconscious equilibrium, where you both intuit your power & speed levels, in yourselves and each other, and then you sync up. Then, the clock beeps, the round is over and you can't believe it went that fast, and you and the other guy have smiles on your faces and you don't know why. Regardless of the weight difference between you two, regardless of what power level you decided to spar at. Could have been hard or light, doesn't matter, because you both went at the same rate. My 2 favourite partners when first starting out? A HW buddy 25kg heavier than me, and an 18 year old kid 15kg lighter than me. The difference was never an issue.
Now some people simply don't believe in that and never will, ever. And they get extremely upset when it's ever brought up to them. So best not to. Just stick with the minority of guys who actually do want to work with you.