I am not sure if this is an OpenLDAP thing or an AD thing.
When I use ldapsearch to get the members of an AD group with more than 1000 members, I only receive the first 1000:
# ldapsearch -LL '(cn=testgroup)'
--clip--
dn: CN=testgroup,<...>
member;range=0-999:CN=test999,<...>
member;range=0-999:CN=test998,<...>
member;range=0-999:CN=test997,<...>
Ciao, Michael.