Yet in the equally crowded Netherlands, the average house size is much larger. Is it just the house building industry is really poor here in the UK?

Yes, the new houses they build are effectively shoe-boxes. Where you see them clearing land and think "ah they'll put a house there", instead they build two.

Pretty much, yes.

The average new-build British home is 76 square metres; a typical single-wide mobile home ranges from 60 to 93 square metres.

We're a crowded island, we have small houses.

Haven't run the numbers, but the impression is that the South of England is much more densely populated. The majority of people lives south of the M62 and east of the M5/M6 highways. And then there's greenbelts to prevent the endless exurbanization.

Good point. Greater London, Hertfordshire, Surrey, Essex and Kent all have a higher population density than Holland, and between them count for a significant part of the UK population.

When you go north, and more generally away from population centres, the houses do seem to be quite a bit larger. There, of course, there are other limiting factors, most often wages.