It is impossible to add an entry larger than 4MB to a PDB backend. For example, adding a static group of 150k members results in this error from ldapmodify:
Attempting to add the same entry to a JE backend succeeds.
NB: the LDAP connection handler being used in both cases had its max-request-size property increased to 10mb.
The thread on the community forums reporting this is at https://forgerock.org/topic/static-group/