I can’t find a specific scientific study that says it online, but common threads are expressed here. There is a reference to a UK study here also, as well as references to many more studies.
Basically, the idea is that too much sleep weakens the body and its immune system because it is not awake long enough to absorb sunlight or to get exercise. Too much sleep seems to be a contributing factor in diabetes, heart disease, obesity, and depression. It can also cause migraines. Also, too much sleep results in an excess of melatonin which leaves you feeling drained of energy.
According to WebMD, too much sleep has been shown in studies to lead to diabetes, obesity, headaches, back pain, depression, heart disease and an increased risk of death.
With some of these, however, it may be the chicken and the egg: do you sleep too much and then end up obese and diabetic? Or do you get obese and then diabetic, and then start to sleep too much?