Index memory usage by GSI Indexes

Currently i have set Index memory as 4GB and index mode as Standard Global Secondary in enterprise edition.
But i would see max index RAM is not using 100% for 4GB. From documentation i have read as Standard GSI can spill to disk when memory runs out. But i don’t see all allocated memory is being used.

What percentage of memory is used in Index Memory.
Do we have any option to set for index to use all index memory allocated.