Well, reporting only on what is used is the same as using independent dimensions again.
No, it's not. Chances are independent dimensions when the items would be an attribute of product would be a design error. This is an ETL choice. But as you point out that it's important to know what hasn't moved, then why are you asking if you should load the unsized products even if it will double the size? It sounds like you know you have to load them. If in the dimension I described I had to report on the 75% unused combinations, there would have been no choice but to load them.
I don't see how including the unsized doubles the data in any case. I would see it something like
Shirt XS
Shirt S
Shirt M
Shirt L
Shirt XL
Shirt US
In this case, the unsized adds one more row to the 5 already there. I'm sure you're real life situation is not a simplistic, but the idea likely pertains.