Many staff who appear to be working for Ryanair (because they're wearing their uniforms) are actually working for handling companies. Those in the UK seem to be reasonably strict about the dimensions of luggage. (After all, the bags must fit into the overhead lockers). But some overseas staff are less so. However they're ALL very strict about enforcing the 'one bag only' rule. (Laptops, handbags, airport purchases, etc must ALL be placed inside a SINGLE bag).
Ryanair keep their fares low by maximising the number of flights each aircraft makes in a day. They only allow 25 minutes from landing to take-off. During that time they've got to get 200 passengers and their luggage off the aircraft, and another 200 passengers and their luggage on again (plus refuelling, pumping out the loos, restocking the galley and carrying out pre-flight checks). They can't afford to have people fighting over inadequate locker space because some people won't play by the rules.
Chris