You may also want to consider a 2-up machine and ride it as a single. It's already set up to carry the weight of 2 people plus it will have more room for someone of your size. Most 2-ups are more expensive with larger motors but there are some models starting with 400cc engines. Currently Can-Am, Polaris and Artic Cat have 2-up machines.

 

All Polaris ATVs come stock with 6ply tires. At least they use to.