Mosquitoes find animals by searching for carbon dioxide. Therefore, the more you breathe, the stronger the signal.
Many insects can also detect heat, which helps them land on their victims. If you’re a larger person, you’re naturally going to be more of a target, since you’re breathing bigger breaths and outputting more heat than a smaller person—and unfortunately, you probably can’t stop breathing. You can, however, wear lighter colors, as some studies suggest that darker colors tend to attract insects.