Is gallons bigger than a litres?

A gallon is about 4 liters (~=3.79 liters).

A kiloliter is 1000 liters.

1000 Liters >> (much greater) 4 liters

Therefore, a kL is much greater than a gallon.

When trying to visualize how much volume a liter has, just remember those bigger bottles of soda are 1 or 2 liters (depending on where you live). It gives you some kind of visual comparison.