I have never seen KLM weighting hand luggage. They may gate check for free if it is obvious that luggage bins are full. Although most of the time I see them accepting everything followed by a frantic and time-consuming improvisation of crew to have it all stowed somewhere (e.g. a trolley of someone in row 18 ends up in a bin in business class). And obviously they prevent trolleys with too big dimensions to be boarded (and that will cost).