Size wise, some are bigger than others. especially when comparing night cabs to a day cab for example. some are taller, but they are all basically the same width (2.55M approx). so they aint really getting bigger.
As for driving up behind you and forcing you to speed up, that's all down to the person behind the wheel. Some are pricks, most are decent. but that can be said for most, if not all modes of transport.
Slow lanes? I take it you mean the overtaking lanes on certain roads? If so, an unladen HGV is totally different from a fully laden one, so you can do the speed limit pretty much up it as a car can. And just because it is a HGV it doesn't mean it has to slow down to allow others to exceed the speed limit. Obviously common sense comes into it on differing roads, as HGVs have slower limits on some roads.
I don't believe they are too powerful overall, mine certainly isn't and its a new truck.