Definition of "slimmest"

Since then we have had the opportunity to travel in them and were much impressed by the internal styling and comfort of the seating (with the minor exception of the single seats which have been tucked in longitudinally alongside some of the entrance vestibules and which are so cramped that only the slimmest of female figures could be comfortable in them; the space they occupy would have been better utilised to increase standing room).

1960 December, “The Glasgow Suburban Electrification is opened”, in Trains Illustrated, page 715