It really isn't anything to do with the size of the house but how many things you have that consume electricity. For example Dh has a laptop and 2 monitors and works from home 3 days a week. Eldest Ds has the same work set up as Dh. When youngest Ds is here and not at uni, he has a laptop and one monitor. I just have a computer and one monitor. None of that would change if we had one less bedroom.
It is all the charging of phones, watches, electric toothbrushes, ipads, everything just adds up. Plus Sky boxes, Xbox or PlayStations, tvs, laptops, plus kitchen appliances, washing machines and tumble dryers, dishwashers, coffee machines, undercab lighting in the kitchen as well as normal lights, outside lights, all just add up. It is a bit like when you go round Ikea and this item is £2 or this is £4 and you get to the till and all of a sudden you have spent £78!
For reference, we use £184 a month in electric, we are in a 4 bed house with 4 reception rooms - lounge, dining, and 2 studies. There are 4 adults living here.