We are learning more about the vulnerability of different people to certain diseases. Why some are Diabetic with just modest excess weight is one of the most important.
Fat tissue is an active endocrine organ - making and processing signals that the body uses to regulate energy sensitive tasks, especially. Female and even male fertility are very sensitive to these changes, but not in the same way for everyone.
PCOS and diabetes risk - characterizing the mechanisms
Obesity Action Coalition review article on Infertility