i think its the way our cars are proportioned and how our cars always gets compared to exotics which are really really wide cars.
Even a RX7 looks wider than ours, and that car is tiny!!! Our cars are really flat sided and the shoulds are not broad from the front so it looks narrower than it really is. There are no fender flares on our cars at all. Here are some real world numbers in width for cars that may look much "wider" than a nsx but is not.
NSX width = 71.3 inches
S2000 width = 68.9 inches
RX 7 FD width = 68.9 inches
EVO 7-9 width = 69.7 inches
STi width = 68.5 inches
E46 M3 width = 70.1 inches
E92 M2 width = 71.2 inches
Supra TT width = 71.3 inches ( everyone calls this car fat looking )
so as you can see, our cars are pretty wide compared to our peers. its only when you start going into lambos and ferraris that we start to get dwarfed but those cars are as wide as trucks
lambo murcielago lp640 width 81 inches
lambo gallardo width 74.8 inches
for reference sakes
First Gen chevy envoy width 71.4 inches ( as wide as our NSX )
2nd gen MR2 turbo width 66.9 inches ( wow much narrower than i thought, we have over 5 inches on this car )