Eddie Bauer is releasing a 1,000-fill puffy in October, and the North Face will debut a collection of Advanced Mountain Kit jackets and insulating pants warmed by 1,000-fill down next spring.

A jacket with 650-fill down takes up less volume than one with 1,000-fill down, so it’s denser and has less space for warm air. But when it comes to warmth, what really matters is how much of ...