I think that, in the Good Old Days, it was more common for people to make their own juice by squeezing oranges or grapefruits. It would take a lot of fruit to get more than a small glass of juice, especially if you’re making some for the whole family.
Then there’s the fact that those juices can be fairly tart/sour if they come from a can, so that a little goes a long way.
Finally, as you mention, there’s cost and sugar content. Servings of soft drinks, too, used to be smaller in the Old Days.