This is one of my biggest pet peeves.

My oft suggested solution:
Have the tags that are printed at the gate spit out a differently-colored tag. At the destination, these bags get sent to the baggage office rather than the carousel, and checked bag fee is collected there that is 200%-300% of what you would have paid at check-in. DL makes money, pax are encouraged to check larger bags ahead of time, and boarding isn't slowed any more than it currently is.

Sadly, it's rare that GAs check for size at all, really. They're focus is on scanning people through the gate, which leaves FAs to do the bin shuffling and sending oversize bags back out to the jetway. And TSA definitely isn't looking for oversized carryons at all. If it fits through the x-ray machine they'll let you waltz right on through.