I think the lack of refrigeration may have something to do with this subject, and these things come from a time when portions were smaller as the contents were a sweetmeat delicacy rather than something you bought by the gallon in Tesco - sat on the sofa with a large tub of ice cream and desert spoon and hammered your way through whilst watching the box in the corner. I think this size is fairly typical of sweetmeats, jellies and custards. I always think they're good to collect, don't take up half the shelf and mostly not too expensive - also a lot of folk don't know their purpose so prices can be reasonable.
I remember being five, and we lacked a fridge in the house, so the only way you had ice cream - for a birthday etc. - was to run half a mile to the shops - run back again, and hope it hadn't melted on the way.

Milk and butter, in warm weather, was stood in water overnight.