When I think back to all the tits I've played with, I'd say that a pair of 32Ds should just about fit into a pint glass (as strange as that image may seem). So concur with the above post.
After looking at an empty bottle of coke, I'd also say that a pair of 34GGs would just about fill up a 2 litre bottle (us Brits use both imperial and metric for measuring volume).
As for the OPs question regarding a gallon of tit, well that depends on whose gallons. The US gallon is 3.7 litres while the UK gallon is 4.5 litres.
For US gallon I reckon a pair of 32Js should be about right, whereas the UK gallon would need at least a pair of 32K/L cup. That's going by looking at an empty 5 litre bottle compared to the biggest pair of tits I've fucked.
You can tell I'm bored as fuck and have way too much time on my hands.