While there is some semblance of sanity in the size and allowance of carry-ons for Canadian airlines, not only is the allowable size of carry-on pieces larger in the U.S. (22in vs. 20 in.), the monitoring of what people were bringing on board was lax to say the least.
With the opportunity for a new cash source the air carriers south of the border have pinched themselves awake and are now putting tighter restrictions on what you pack into the overheads and under the seats.
In the past it really had gone much too far almost to the point of ridiculousness as people argued with each other as to who owned the real estate over their heads for the duration of that flight.
By being more restrictive the carriers know you will be forced to pay for that first bag in check in. It’s new money and “follow the dollar” is the new mantra of airlines north and south.