Or, if you instead make the image larger on your screen, it's still 2 megabytes, but now you may start seeing the image start to pixelate more.
So, for example, if you have an image you upload that was 2 megabytes in file size and you grabbed the corners to scale it down, the file is still 2 megabytes. You're only increasing/decreasing the image's physical size. By resizing an image after it's already been added to a Canvas page, you're not doing anything to the file size. When you insert an image into a page of content in your course, you can grab the corners/edges of the page, but in my opinion, that's not what I would recommend doing. Hello don't know of anything available in the current Canvas toolset that would allow you to resize images all at one time (or even on one screen), nor do I know of any way via the User Interface that you could set a default size for your images.
