IIRC, metabolic rate drops somewhat during sleep, but I don't think "too much" sleep in and of itself would make one gain weight. Overconsumption of calories for one's activity level would still be at fault.
thats why breakfast is so important. if you dont eat breakfast your metabolism slows down all through the night and until u eat next after u wake up. so the most important meal of the day is breakfast, cuz it jump starts your metabolism and gives you energy to start your day.