Wikipedia says,

  Porpoises range in size from the vaquita, at 1.4 metres
  (4 feet 7 inches) in length and 54 kilograms (119 pounds)
  in weight, to the Dall's porpoise, at 2.3 m (7 ft 7 in)
  and 220 kg (490 lb).

Doesn't say what is most common, though, so if 30-70 kg is more common, I'll take your word for it.