The width limit of 2.5m is common across most operators because it's the width used to plan the ships loading lanes. Basically freight vehicles used to be 2.5m wide (although in some circumstances they can now be up to 2.65m). Thus the lanes are marked on ships decks to allow a 2.5m vehicle plus room for a (slim) driver to get out from either side. Anything above 2.5m wide risks overlapping into a second lane, consequently taking up two vehicle spaces.
Hope that helps
Hope that helps