Unless and until baggage size checks are taken away from the airlines and made by a 3rd Party - Group4 or some of the other friendly, welcoming security organisations we know and love - people will push their luck
Ever see the "Excess Baggage Family " sketch on "Goodness Gracious Me" a few years back?
and of course no-one will do anything about duty free or air-side purchases
the only answer would be to stop people taking ANYTHING into the cabin (which I think happened after 911)