Obesity affects the quality and frequency of your ovulation. Obesity can affect the hormonal balance that regulates your menstrual cycle, which can in turn cause more cycles to be anovulatory (the ovary doesn’t release an egg in such cycles).
In addition, even when obese women do ovulate, the quality of the eggs is reduced.
Both of the above factors contribute to lower fertility in obese women. Losing even 5 percent of your body weight can increase your chances of getting pregnant.
Source: Williams Gynaecology, 3rd Edition