We already resize down to 100x100, we use the same image for images on the main page, in discussions and user profiles. Storing a unique copy for every page would drastically increase the amount of data stored on the server. 100x100 is a fairly good compromise and honestly 674KB is already fairly small. An additional 200Kb saved won't do much in most circumstances.
Further unlike HTML/PHP only application because flarum is SPA once the initial pages load the only thing that gets loaded after is the JSON from the API and any images being served. CSS and JS will have already been cached by the browser in most cases as well since we do proper cache busting/serving.