There are a couple of reasons the print may look different on the mock up vs. the actual printed mug:

1. The mock up is on an 11oz mug.
2. Our max print size for both 11 & 15 oz mugs is 3 inches x 3 inches.
3. This design is taller than it is wide so it will take up the 3 inch height but does not fill out with width as much because of the tall/rectangular composition.

Another article in the FAQ section regarding print size on mugs can be found here:

Attached are photos of both 11oz & 15oz mugs which illustrates the print size and area.

Please use these as guides when you are mocking up your product; it will show your customers a more realistic mock up of the actual product.