I recently had my legal BA size cabin bag on a trip where I was on LOT. The first leg was fine as it fitted in the overhead locker (BA and LOT have the same bag size limits). On leg 2, an Embraer, the bag could not go into the locker although I was permitted to bring it on board. In the end they strapped it into a spare Y seat. Not my fault If the locker's too small but the size limits remain across the board.