Flew LGW - MAD this morning. Saw lots of passengers with hand luggage quite a bit larger than the standard box posted at all check in counters.
On the way out of the plane I asked the FA about this, pointing to two backpackers (this was an EZ flight) with packs at least twice the permitted size.
She said that it all depends. Some people sail right through and others are turned back to check in some items. She also said that some people bring a nearly-empty bag into the secure area, then fill up with duty free.
Somehow the pair I spotted seemed more headed to a beachside campsite than hearty duty free shoppers, but you never can tell.