Niko Neugebauer, in part 73 of his ongoing columnstore index series:

This result was observed right after the finish of the loading script, where we can clearly see 4 Delta-Stores for 10 Million Rows. 3 of the Delta-Stores are Closed and 1 Delta-Store is Open, which is an absolutely impossible combination if we think about Clustered Columnstore Indexes, where one would expect to have 10 Compressed Row Groups or 10 Delta-Stores (9 Closed & 1 Open).

If you take a more detailed look at the associated sizes of the closed Delta-Stores, you will see that they increase each time a new Delta-Store is being used. For example, the first one is capped at 1.048.567 Rows, the second one is capped at 2.097.152 and the last closed Delta-Store is set to 4.193.904 Rows – meaning that the size is being constantly doubled.

I’d like to see this as the first step toward expanded sizes for compressed rowgroups.