When using GIMP's scale option on a selected image, the entire layer is scaled. Canvas scaling also scales the layer (along with all layers). There are also Transform tools that can scale, unfortunately they also scale the layer, not selection. My workaround is a bit sloppy. Fit canvas to selection, then use scale to desired size, copy and paste back into its original source. Scaling down is similar, but with fewer steps. GIMP is such a great program, I don't know why scaling a selection is so weird.
If you have a selection, it will scale only the selection. You have to use the Scaling tool (
Shift + S +
<click> on image), not the "Scale Layer" menu.
If you want to use
Scale Layer for some reason, convert your selection to a "floating selection" (which is more or less a "modal, temporary layer") first, by pressing
<ctrl>+<shift>+l after you create a selection. (Or using th menus:
update Pay attention that in the options for the scale tool (showing up in the tool-options dialog, beneath the tool palette in the default layout), allows one to choose which is the scale tool target, the
Transform control, with the options to operate on the "layer", "selection", "path" or "image". The usual behavior as described above is the one you get when "layer" is selected.
According to the behavior you describe, it is likely "image" is selected in this control.
The Unified Transform Tool (shift + t) intuitively scales a selected image similarly to the way photoshop works. Make sure to have the layer icon selected in the tool options tab left of workspace. Make your selection, hit shift + t or click on Unified Transform Tool, click image in selection and handles (squares) will appear for dragging. The handles (squares) in the center of the selection's sides can be dragged to easily scale. The corner handles (squares) will skew the selected image.