Giantess/Size Difference
One of the first, from the Stable Diffusion 1.5 era, and still among the most downloaded: a size-difference concept trained on art of a small boy with a giant girl. The author's own notes warn that it is unstable and picky about checkpoints. Here as much for the history as the results.CivitAI
Giantess Shrunken Man Handheld & Palm Play Gentle