It unusual for males to have big dewlaps - so check sex if unsure. Otherwise it can be weight related but doesn't necessarily mean they are currently overweight. Sometimes when a previously overweight rabbits loses the weight they are left with a dewlap still. Have a feel of it, if it's fat it should feel thick when you pinch it, if it's just skin, it will be very thin, just a pinch of skin with a lot of fluff making it look like fat.