Got to agree with oz on this. I feel airlines' enforcememt of their own rules is much too lax.
When my wife and I travel, we always check in one large suitcase and bring our flightbag on board. When filled, my old reliable flightbag has the small and large pockets fully expanded, and has never failed the "sizer" test. If it's heavy, it goes under my seat, rather than endanger someone, should it fall out of the overhead. Many times, because of its apparent bulk, it has drawn glances from the gate agents and FAs, but its never had to be checked, as it fits within the sizer's parimeters. And we usually get a "cabin baggage" sticker from the gate agent, after its dimensions have and weight been verified.
But it never ceases to amaze us what we see being brought aboard the aircraft. Large softsided hockey equipment bags fully loaded,
those large rolling "flightbags" much too large to fit in the sizers let alone get into the overhead bins, yet being stuffed into the overheads, much to the squashing of other people belongings.
The airlines can make all the rules they care to. What it boils down to IMHO, is that these rules have to be enforced, and travelers should be made aware of that fact.

bj-21.