It can be a "crap shoot," and varies from company to company and tile to tile. It is not unusual for different size tile, even in the same dye lot to vary a bit. Smaller tile is often just "cut down" larger tile. Examining the back will often show this up. The smaller tile may not have a "complete" border etching...indicating it was cut after firing. This is done often with bullnosed tile.

Chances are that these tiles were from different "firings" and may well very. If the differences aren't huge, you might even like the subtle shade variations. Your call...