If the global index appears to be growing too large, you can reload from the backend directory servers. Use the reload-index tool with the --fromDS option, overriding the configuration of the prime-index-source property. You can do this on a one off basis if the global index appears to be growing too large as follows:
$ bin/reload-index \
  --bindPassword password \
  --baseDN "dc=example,dc=com" \ 
  --fromDS