Short answer: The Americans
Long Answer: SHW's never really took off here in the UK as the main focus for Robot Wars was the Heavyweights, why build something thats more expensive and wouldn't get on TV? Whereas in America they had Battlebots which actually offered SHW events, and the Americans being Americans a few decided to Supersize their machines. Although even over there they were somewhat of a rarity. I believe the only SHW i know of in the UK was (i think) owned by Craig of Anto fame?