No they're not all that big compared to aftermarket brakes (KAD, Endless, Brembo, GReddy, etc). It's just that that most j-spec Supras come with the smaller brakes (2 pot front, 1 pot rear). The upgrade from j-spec brakes to UK spec ones is a popular upgrade as the UK spec brakes are remarkably good, better than almost any other car has as stock. This is the reason why you will notice so many posts asking whether this or that wheel will fit over UK spec brakes - because the owner wants to upgrade to UK spec brakes after he already has the wheels he likes.
Also, upgrading to UK spec brakes is not usually classed as a modification by insurance companies, seeing as the larger spec brakes could have been fitted as (maybe as an optional extra) from new.