I've seen similar things at a few races. Firstly Andy Moore, who used to organise Burbage Skyline gave a prize for the first person over 13 stone. I think it was 4 pints and he normally won it.

Secondly, there was a prize for the first person over 200lb (about 14 stone 4) at Ted's birthday race: runners and riders this year. There was a set of scales there for people to prove it at the registration desk. When I queried it the guy said it's a common thing in the US. I think, as with Andy Moore, he was looking forward to taking the prize.