Actually the 70/45 has a larger volume then the 68/45. It's "smaller" in that its shorter, but it makes up for it in being fatter as it's width is greater. The 68/45 appears to be "larger" in that its length is longer and its width is shorter. That's why there's a difference in size, but each tank either makes up for shorter length in a wider width, or longer length for a shorter width.

Just my 2 cents, correct me if I'm wrong.