Is it true that the image size and the canvas size are the same thing in a photoshop file?

Yep but for the usage you use image size to decrease or increase the image with its content. The canvas size is used to decrease or increase the size of the canvas area you are working on. For example if you have a600x800 pixels file and you changed the canvas size to600x400 pixels, it will "eat" from the content inside the file and whatever elements on the edges will disappear. On the contrary, the image size when you decrease it, it will preserve everything inside the file and decrease it altogether.