Answer :

1 gallon is equal to 3.78541 liters. Therefore, a half gallon would be equal to about  1.89271 liters. Thus, a 2 liter bottle holds more. 
1 gallon of milk is 4.5 litres ( rounded ). Half of 4.5 is 2.25 so gallon of milk is more.

And by the way, milk has more mass than juice, just a fact.
 
If you wondered where I got 4.5, just search into your search engine of choice.