the fit to box setting has nothing to do with the div that the image is inside of.
to the end user, it looks like image resizer is doing the same as adding the width or height setting to the image tag, but what is actually happening, is that the server is dynamically creating a new image file, resized to the settings of the width and height on the image tag.
when you don't use image resizer, the original image is sent to the browser, then resized based on the height and width settings. This can cause a lot of band with and slow load times if you have really large images.
With image resizer, the server creates a new image based on the width and height, and this is sent to the browser, this results in a smaller download, and faster load time.