either modern garages have increased in size or it must be a double
They've increased in size - it's been a number of years, perhaps even decades or more, since houses with garages were provided with garages of a size you couldn't put a car in. Most houses we viewed that were built between about 1985 and 2000 had garages which, as you say, were too small for using for cars but not much since then.
Most new build single garages now measure about 3 metres x 6 metres, which is easily enough for any modern car (Though you'd then lose the ability to store much else in the garage).
It's probably something to do with the planning requirements that mandate a certain volume of things like car parking spaces, cycle storage etc and it's easier to make the garage large enough that it counts towards some of those requirements than it is to provide additional parking at the front (Even though despite this most people still don't use them and instead just park on the pavement).