What is the difference between coach & first when it comes to airplane seat belt length?

I am surely a big guy, I cannot fasten my seatbelt when sitting in coach without an extender. When I fly first class USA domestic I fit fine. Just wondering the difference in length of belt? So now I am curious, at what dress size of person does it begin with who cannot fasten their belts?|||they do fit different even according to class now that u mentioned it; or else i would never have noticed. the times i also have to travel to sit in ecomomy those belts were a bit tight and when i trvl in first i dont' even noticed those seat belt problems at all. what a great question, i will have to ask around myself as well, thanks for bringing it to my attn as well.