It might be an OH&S issue. I certainly in my instructing days would have been concerned with hygiene issues in the often cramped cockpit of light aircraft, sitting next to a larger student. Add a hot day into the mix, combined with performance issues and body odour, and well, you may lose willing instructors to put up with it.
For your own sake, lose some weight, adopt a healthy lifestyle and that PPL will be far sweeter when you get it.