Distribution lists in outlook have a size restriction that will not allow you to add more members once a certain limit is reached depending on how big each contact’s information records are.  A work around is to split the distribution list up into multiple groups, or what I did was create a distribution list in AD and allow the user “owner” privileges on it.  After adding members, you can also remove it from being listed in exchange global address book and add the global DL contact to their local contacts list.