But each manufacturer gets their own limit based on the type of vehicles they make.
If you only make small cars then you have a much lower fleet limit than if you only make HGVs, otherwise it wouldn't be fair.
Toyota might have the lowest actual average, but also a cripplingly low limit too, as they only really sell passenger cars in EU, but nearly everyone else sells vans. I could be wrong, as I cannot find any exact limits set for each manufacturer.
It seems unlikely that they couldn't find space to fit a GPF...