Large number = lighter weight.

Two 15-degree bags will be equally warm, whatever the insulation (assuming the "15-degree" rating is objectively determined, not just marketing hype or wishful thinking.) For example, a synthetic 15-degree bag will be just as warm as a 600 fill or a 900 fill down bag. However, it will weigh a bunch more than either of the down bags, and the 900 fill down bag will weigh less than the 600 fill down bag.

Large number also = lighter wallet. That 900 fill bag will weigh less, but it will also be significantly more expensive. As in, "That's OK, honey, I'll get the mail" so you can get to the credit card statement before she sees what you did. <img src="/forums/images/graemlins/tongue.gif" alt="" />