Hi Tom
It's probably down to history,
Most older working class older houses were never built with kitchens as we know them now, the back room usually had a large coal fired range which all cooking was done on and the only source of hot water, food was prepared on a table in front of it , a smaller room off this room was usually known as the scullery where cold water and a sink was found and often a pantry ( a cooler room for presihables ) over time the scullery became a kitchen with oven sink fridge etc and the older kitchen became a dining room or living area, this led to smaller appliances being popular.
These small fridges initially had a top freezer section, which held little but those of us who remember them will know it was usually ice cream ice lollies, peas as frozen food in the 50s 60 s was a new phenomenon. Even today theses little units are not much smaller than the traditional fridge freezer that most homes have , simply one stood on top of the other. They hold a surprising amount and I know several families who use this size fridge. The other thing is that what we keep in a fridge may be less, originally milk was delivered daily on the doorstep so you only needed 1-2 pints, now we buy 2-4ltrs in plastic cartons from supermarkets, wines and beers have increased so fridges grew to hold those.
I know my friends in Australia put everything in the fridge not only because of the ave temperature but to stop ants flys etc finding them, vegetables etc were often stored in racks under the sink over here.
Another thing that has happened is that often people will have what seems small to you, fridge and freezers in their kitchen with a larger freezer or spare fridge in a garage.
Plus lastly the rise of ready meals and eating out means fridges only keep food for a day or two rather than a weeks worth.
The rise of the American fridge freezers with side by side doors has happened over the last few years more out of staus than perhaps need, retailers over here have signs explaining to customer to check whether they will fit or even be able to be carried through the door easily as many a delivery driver must have got stuck with one trying to fit it in our smaller homes!
I posted a pretty standard fridge freezer that most home would use in the uk. They only take up the same foot print as a dishwasher but are taller. Some even taller still and hold a surprising amount, but quite often the same size as an invidual fridge shown on Doc Martin
Richard
