Container queries are a red herring here; they don’t change anything at all. The question being asked here is “why doesn’t it just use the image’s size?” and the real answer to that is that you don’t want to wait for the CSS to all be loaded before fetching any images at all—that’d be a pessimisation, not an optimisation. Hence the sizes attribute cares about viewport sizes, and always will do. Container queries will almost certainly never get involved in this, because, along with the rest of CSS, they’re evaluated after image loading begins.