Coming to the point : To be honest with you, this is not a NetApp limitation. This is a NIS protocol limitation. NetApp is just implementing it.
NIS Protocol: NIS is the current name for the service originally known as Yellow Pages (YP). NIS and YP are functionally identical. NIS is originally part of the Network File System (NFS) software package that included commands and daemons for NFS, NIS, and other services. However, each is independent and each is configured and administered individually. NIS protocol limits maximum size to 1024 bytes. This limitation applies to all NIS map files.