I can say that when I was on the field, one of the most irritating things was watching the people fall behind not because they weren't talented, but because they chose not to get in shape before tour. This goes for skinny and fat people alike. Drum corps is a very demanding activity on the body, specifically the joints. If you have not been training and strengthening your muscles surrounding those joints, you will be in a lot of pain and possibly get injured. And I will say that those of us who took the time to get into shape in the off season like the staff said to starting in...oh i don't know.....November, it is very irritating to see that.
I also think there is a difference in being fat and being out of shape. I have marched with fat guys who can 100% hold their own on the field and keep up with everyone else, they just happen to be fat. Now, these guys were also "in shape" during the winter camps, and the visual staff could see this during the "demanding" visual blocks and physical training sessions we held at camps. If you are a big guy or girl and can play your butt off, that's great. But, you better come into those first camps in some sort of physical shape and able to get through vis block. The staff NEVER believe what you are "going to do to get skinny" because clearly you haven't done that already.
Also, some uniforms are designed where just about anybody of any size can look good wearing it. Others....not so much. If you are going to look like a mess in the uniform, they're probably not going to put you in one. That uniform represents a lot more than just you wearing it, and they aren't going to let one person ruin that look just because he or she can play really well...