Indeed,they were low. The 737 needed the extra height to accomodate the engines. The DC9 with the engines mounted high at the rear had a lower fuselage,which allowed the bag snatchers to load & unload from a barrow. Refuellers needed no special climbing structure. Also with the engines at the rear,DC9s were known as the 'whispering T-Jets'. (Please note; the word 'whispering' in this context could be substituted with 'ear splitting,crackling')