With outpainting it is possible to extend the surroundings of an image. The actual (file)size of the image gets enlarged here as well. If only the forest should be extended, and the initial prompt is “a house in a forest in fall” then the outpainting prompt is only “a forest in fall” (or otherwise it would add houses).
With inpainting you can take a large image as a base, mask part of it and change or add something. In this case I added a pond in front of one of the houses (by adding “pond” to the text prompt).