Unusually Small Doors

Today I worked with Dad and my sister Catherine in Canary Wharf. On the train back Dad handed me his Kindle (which I'm really jealous of, by the way) and showed me an interesting fact.
We've always seen the tiny doors in Petworth on the way to my university, they hardly reach your waist some of them. We always assumed that they're so small simply because, then people were small. Turns out they're so small just because in those days doors were expensive (like windows) and so the smaller they were, the less money it cost to make them.
Sounds far more logical than our short-people theory. After all, stooping is free.