Compactions happen on the tablet server. They only write data. By definition, they increase disk usage, right away. Only the garbage collector, which needs global state and thus runs on the master, can actually delete files in Colossus and bring down disk usage. It doesn't always run as frequently as one would think or hope. It definitely doesn't run immediately after one tablet has been compacted.

I remember one incident where a team got an alert that they were at 90% of quota or so. They compacted data too aggressively and they actually hit 100% before the GC could do its job. That's why I mentioned that you should compact a fraction of the table, then let GC run. Google being Google, I'm sure such a tool has been written, in several variants, by N different teams.