if you're overweight you can still get in....you just have to have the right BMI, or percent body fat... I don't know the exact formula...but it involves measuring the circumference of your neck and your waist (at the belly button). The BMI is different depending on your age... but for me (I'm 6'0, 235lbs, 28 years old), I need a 17" neck and a 41" or 42" waist to just make the cut. thats for the army, not sure if the marines are different.
so if you're pretty muscular, you should have a pretty big neck...all you gotta work on is losing some off the gut.
Or, you can always do the "arms test". it involves doing something like step aerobics for 5 mins at a fast pace