The sheets could be like... giant ghosts!
...
...Don't judge me!
Also, I think that the possibility of the mirror leading to a different world/area makes up for the attic not being bigger. Why would anyone want to buy so much junk that they have to end up dumping it in a room to gather dust and cobwebs anyway? Does anyone actually keep their (not empty) attic clean or do they forget all about the contents inside except every now and then when the memory resurfaces in their mind?