Why do large prints cost more you might ask? It's not the extra ink used as you may have guessed.
Large prints mean more setups
The extra cost is tied to the extra setups often necessary with large prints. A setup is the set of screens needed for each print position, and the time it takes to prepare them for printing. If you'd like your print to fill as much of the shirt as possible, the screens used on size Small won't achieve your desired result on size XXL - as one example.
That means that two or even three setups, each sized for a range of t-shirt sizes, are required to maintain a large print relative to each size.