Average (well, median) US house size: 2000 ft^2
Average UK house size: 68 m^2 = 740 ft^2
That makes for a very, very different HVAC bill.
Numbers vary a lot by source but the relationship (2x+ larger in the US) holds. For example: https://www.zigguratrealestate.ph/post/how-big-is-a-house-av...