It saddens me quite a bit to see the in game rules being mandated by what once were out of game minis![]()
No winning this one as you don't want minis involved in any way with gaming (I assume), but minis have been a part of the hobby for ages and it makes perfect sense to me to design monsters based on what's available (WotC or otherwise) to depict that monster.
Also... fights against larger creatures are harder to depict on limited grid space. There is no denying that D&D 4e is heavily tied to the grid (and I respect that some people dislike that) so it makes sense to make the majority of your creatures sized to allow for dynamic grid combat.