The biggest guy i've fought was 6'6" and about 220 lbs (fair amount of flab though). He had about 20 fights under his belt at the time...I'm 6' and was about 155 (a very lean 155) at the time. The first round i pretty much got my ass kicked because i couldn't measure the distance he could hit me at very well (his reach advantage was obviously huge). 2nd round i realized that i needed to fight inside so i cornered him and avoided his punches, got inside and worked some good left hooks and body shots. Eventually i caught him with a solid left hook and he crumbled onto me, at which point i landed a solid right uppercut to the chin and he was out.
Although i did win the fight, the SOB did break my nose in the first round. Also in case no one figured it out by the huge weight difference, this was not a sanctioned fight.