You have a volume V1, and let d1 be the maximum density of information you can store in V1. If you try to store any more, your storage will collapse into a black hole, so d1 is the maximum density.

Now you get 8 times more data. But cannot story this data in a volume 8*V1, because if you try to use 8 volumes V1 with density d1, and place these next to each other, they will collapse into a black hole.

Thus, the more data you have, the smaller density you can use for storing it.