Is this procedure new?
A few weeks ago all the three passengers directly ahead of me were told was "Make sure it doesn't happen again" and they proceeded to board with their huge carry-ons plus duty free bags. I suppose achieving an on-time push back was of greater priority than enforcing the rules.
These passengers claimed not to be aware of Ryanair rules. Being officious, I pointed out that all this was clearly printed on their CheckNGo boarding passes. Either they hadn't bothered to read what was written or were "chancing it".
My carry-on fits under the seat so I find the mad attempts to squeeze quart bags into pint spaces overhead a hoot!
Obviously, with Web check-in the problem arises because there can be no check on bag size until passengers are at the gate, if at all.