Actually this isn't true. There are a range of styles but what is now the kitchen/diner in 20s/30s houses is achieved by removing the wall between what was usually the bathroom next to the kitchen (not toilet, that was separate and often had a door facing outside) as upstairs plumbing is a fairly recent thing. Fitted kitchens weren't invented until later. So when the bathroom was moved upstairs, the wall would often be retained and used to separate a kitchen (where the bathroom used to be) and dining space (where the kitchen used to be).
In the pre-1910 houses, the whole back aspect would have been the kitchen/diner in a 2 up 2 down because it was a working class house and working class people didn't have or need a separate dining room. They would have spent all their days in the back room and the front room was usually the parlour for if the vicar visited (or, it would start life as a parlour and end up as the place where a chronically ill person e.g. a widowed ill MIL or a child with TB would be installed pre-NHS).
In a house with a rear projection (not an extension; they were built like this to enable the middle room to have windows), the back room would be the kitchen, the middle was the dining room and the front room was the parlour, and the dining room would be where the family would hang out between work and sleep.
In slum houses built in a similar style to these (and largely cleared these days but you still occasionally find one in a bad state somewhere) none of the rooms would have had running water, kitchens or bathrooms, people cooked on the fire in the room they slept in, and all of them would have contained at least one family.
If you're interested, the house in Peepo shows the layout of the traditional three-up three-down (as it was used in the 1940s) quite well. But often in working class houses, the kitchen/diner was the only place to sit down (on straight-backed chairs, around the table where you also rolled out your bread etc for cooking, although of course you'd stand up for that to leverage your body weight)!
That changed after WWII when the social class divides started to break down and working class people were less beholden to others, but even up to the 70s or 80s some people maintained a parlour and a kitchen/diner in a two-up two-down.
If you think about the things you'd need to actually cook in a kitchen before fitted kitchens and appliances were invented, you can see easily how much more space a kitchen would need to take up.