Images are served via http request from the client to remote image, not from the PHP code (unless your PHP code is a script of type image). So images are fetched after the browser retrieves the page source, and then it goes out and remotely fetches anything in img tags or in the CSS that's an image type.
ob_get_length is showing the total number of bytes in the output string, and does not do anything to read external images. So no, adjusting the image size won't affect the size of the output string. The only exception there would be if you use a data uri scheme with the img src, so you actually give it the base64 encoded binary of the image. That would affect the size of the output buffer.
header('HTTP/1.1 420 Enhance Your Calm');