well i'm pretty big and it sucks for me.
like you might think, being heavy is good in certain situations. not many in my gym can escape my north south when i manage to get it. it helps with any sort of pressure assuming you can control the other guy's hips and stay off of your back.
i get swept a lot since my center of gravity is fairly high. honestly i try to play a bottom game most of the time since i have a decent closed guard and half guard.
i'm not ultra fat, but i'm definitely too big for my own good. my knees and ankles take a pounding. it hurts my ability to progress as fast as i want to and my risk of injury is higher. hell i just got back on the mat recently after tearing up my knee in september.
that said, i have been losing weight at a rapid clip since i started training. the lighter i get the easier everything gets, not just on the mat. BJJ has given me the kick in the pants i needed to finally focus on getting healthy.