Use an Extended Directory as a Mobile Directory. Since you need two Mobile directories catalogs to do firstname lastname and lastname firstname searches, try using an Extended Directory Catalog instead.
The Extended Directory Catalog was designed for Notes servers and utilizes the Public Address Book template, so in essence, it's a NAB with only groups and people records.
Our two NABs at 120MB and 50MB were aggregated into a 22 MB extended dircat.
Not the 1.5 MBs that a mobile dircat would be , but the extended dircat is more like how users are used to using type-ahead and searching.
I use it for Directory Assistance and a Mobile directory--that's right, one WW directory for 2 companies. Much easier and more flexible than the mobile dircats.