Grant Fritchey demonstrates how Query Store allocates disk space:
I love the questions I get while I’m presenting because they force me to think and learn. The question in the title is one I received recently. The answer, now that I’m not standing in front of people, is easy. Of course the space is not pre-allocated. Query Store tables are just system tables. They have a limit on how big they can grow (100mb by default), but that space isn’t going to be pre-allocated in any way. The space will just get used as and when it’s needed, just like any other system table. However, don’t take my word for it, let’s prove that.
Read on for the proof.