In reply to The_flying_climber:
There isn't actually a "universal" size measurement for bag capacity which gear companies use, so one companies 30 litre could be another companies 35l.
My mate used to use a 45 litre Arete which was of a similar size to my 35 litre pack. He now has a Montane bag which is 40l I think and is of a comparable size to my Lowe Alpine 45/55 litre bag.
Take it all with a pinch of salt.
Post edited at 13:45