Is it possible that the big bezel on the MacBook Air displays is there for structural reasons? Perhaps the lid is so thin that the additional rigidity is necessary?
Might also be related to the base, not the lid. If the base was already as small as it could be engineered, but the screen size was fixed for supplier reasons (i.e. there was no (11.6+x)” screen they could use at the desired DPI), then the disparity between screen size and base size would result in the relatively oversized bezel.
Might also be related to the base, not the lid. If the base was already as small as it could be engineered, but the screen size was fixed for supplier reasons (i.e. there was no (11.6+x)” screen they could use at the desired DPI), then the disparity between screen size and base size would result in the relatively oversized bezel.