In the 50's you'd buy it in an 8-oz can. The sizes we're getting out of vending machines are pretty absurdly large for something that basically started out as a dessert you'd buy to top off your small sandwich and cup of water for lunch.
Everything bigger is clearly meant to be consumed over multiple sittings or by multiple people.
I will grant you that fast food restaurants especially and restaurants in general do offer ludicrous amounts of soda for a very low price.
But the retail packaging has not really gotten larger, in a very long time.
The same thing happened with crisps (potato chips). The small bags that were standard when I was a kid in the 90s can now only be found in the big multipacks, while the standard size is now the old "grab bag", which is 2+ servings. Of course, since you can't buy the old size as separates anymore, everyone eats the whole grab bag these days.
I don't think the whole "multiple sittings or multiple people" thing holds up in reality.