size difference
An image that focuses heavily on the size difference between two or more people; often minigirl, miniboy, giantess, and giant male pictures fall into this category.
If the 'natural' difference in size between characters, ie. 'short' vs 'tall' is the focus of the picture, use height difference instead. If one person has a small body overall, they are petite.
Focusing exclusively on breast, penis, or muscle size difference is respectively breast envy, penis size difference, and muscle envy instead.
For an image that has been changed from its original size, use resized.