Hmm, interesting to try and pinpoint the difference between "lein" and "erl". I would say they are both diminutive but not exactly in the same way.

erl ... would be more related to the actual size

  Sack -> Sackerl (bag -> small bag)
  Wagen -> Wagerl (cart -> small cart)

lein ... would be more related to the inner size (sorry I can't frame this into words any better)

  Frau -> Fräulein (Woman -> Younger/Fragile woman (unmarried))
  Wagen -> Wäglein (Cart -> smaller but also lesser cart)

edit: referring to the answer of pavel_lishin. Yes "qualitative" modifier was the word I am looking for.