If page size and weight was used much more heavily in SEO rankings that would make a big difference.

Some engines ("some") have an interest in promoting sites with advertisements returns.

Should be pretty self-reinforcing. If sites are cropping up that load instantly, then sites that take 30 seconds to load and are sluggish to scroll start to look very bad by comparison.

Penalizing the websites that are heavy.

I'd love to see duckduckgo to add toggleable site sorting by their common page sizes without media as a second priority, at the least.

In some sense this already exists (via users clicking away if it loads too slowly). So maybe the market has a nice balancing function for this already.