Windows must query for the class type of each group member to assign an icon. This query can take a long time as groups grow in size; this is the reason for the default limit.
You can override this behavior in the registry. To do so, modify the GroupMemberQueryLimit REG_DWORD entry in the following registry subkey:
Note The Windows key and the Directory UI subkey may not exist. Create these subkeys if necessary.
Also, the size of the query for member class types is set by the GroupMemberFilterCount REG_DWORD entry in the same registry subkey. This entry defaults to 35 members per query. You can change this value to change the number of members that are returned by a query.