An image file's size on disk is roughly proportional to how many pixels it holds, so halving the width and height — a quarter of the original pixel count — typically shrinks the file by a similar fraction. Resizing means resampling: calculating a new, smaller grid of pixels that best represents the original image, which is what the browser's image codecs do natively.
This runs entirely in the codecs already built into your browser, which is why the whole thing takes place on your device with no upload step and no server-side queue.
Your file is never uploaded, never queued on a server, and never deleted from one — because it was never there.