I was 210 lbs at 5'10" when I entered OCS. Even though I had been running before I got to OCS, the running there was brutal on my knees. You will be running betwen 2 and 5 miles 5 days out of the week at a 7-8 minute per mile pace. The DIs are running machines and they expect you to stay with the pack. Being heavy can be a burdon for the amount of running they put you through. By the time I graduated OCS, I was down to 185lbs. I wasn't fat to begin with and I lost a lot of muscle mass due to the rigoes of OCS, but the weight loss certainly made running a lot less painful on my knees.
There are exceptions to bigger guys regarding PT. My former roommate was 6'3" at around 230 lbs. He was a bit on the chubby side, but this dude could run the 1.5 miles in under 9 minutes. He was a boxer before coming into OCS and his body was already conditioned for road training. Heavy guys can do this stuff, but I think the lighter guys might have an easier time if they are not in the best of shape.
As far as going Marines...you have to do pullups. It is easier to do pullups with 160lbs than it is with 200 lbs. I know a few guys that are over 200lbs that can crank out 20+ pullups, but they train and train and train to be able to do that. I think it would be easier to do the exercises coming in with less weight to pack around, but ultimately it is up to you.
Don't let the PT requirements persuade you to choose a specific service. Join a service for what it has to offer and how you'll benefit it by being a member of it.
"The object of war is not to die for your country but to make the other bastard die for his." - General George Patton (1885-1945)
