Depending on routes. Once we flew to Sofia from London by paying extra for large carry on luggage. There was no check or anything and people brought duty free bags without hesitation.
Recently we flew to Debrecen, Hungary from London. We assumed they would not check as a carry-on luggage is slightly bigger than small with our last experience. At the gate, there were two staff checked bags strictly and we were forced to pay for "upgrade."
For that reason, it is better stick what they say on the website. It is embarrassing and annoying at the same time if they pick on you.