After SERVER-66690 and SERVER-66685, for high cardinality cases, we can run into WT cache pressure if we let buckets grow to the default size. In the case where we have a large number of active (open or archived) buckets and we allow the full default bucket size (125KiB), we could easily exceed the WT cache size and get into a state where each document write will trigger a write to disk.

Instead, we should track the number of active buckets and lower the size limit dynamically so that all of the active buckets take up no more than half the WT cache. This will generally prevent us from getting into a constant write-through pattern, at the cost of slightly worse disk usage and query performance (due to smaller buckets). This offers the users at trade-off between instance size (RAM) and disk size.