As Tom said, the fuel you take in is of the utmost importance. However, there is another aspect and that is the duration that you exercise for.
No matter how big or slim one becomes your heart and lungs can only be so big and their ability to pump oxygenated blood to demanding muscles is limited. When a large muscular individual forces their heart and lungs to work at a high rate for extended periods of time (45 mins +) they run the risk of the body starting to eat away at their muscle mass, this isnt because it isnt getting enough food, but because the body is trying to reduce the overall size of the body so that the heart can get oxygen to the right place without having to feed a lot of beef cake that isnt doing much. The bottom line is that unless you are one of those freaks (Like Navy SEALS) that are blessed with a rare muscle type that has the power of an ATP athlete but have cellular respiration like a marathon runner you are going to have to either make a choice about how much weight you want to give up.
We have another thread here in the Clyde section on nutrition, but it will be going into natural ability, determining your own natural ability and what you can and cant reasonably expect given your make up (be you a fast or slow twitch kind of guy). Its not all about trying, somethings are impossible for some folks. I hope to get into that as soon as we have the important stuff about grub out of the way
