Nagios plugins should only return one line of output. Any more than that and there's no guarantee of how it will be interpreted or displayed. Additionally, there are differing size limits on several of the components used in Nagios XI. The Nagios Core monitoring engine will only read 8K of output, including performance data. NDOUtils will only read 4K, not including performance data. Nagios XI has its own limit for display purposes which I can't remember right now, and I'm sure there are other components I'm forgetting about. It's a bit messy, but if you follow the developer guidelines
you shouldn't run into problems under normal circumstances.
You could write the additional information to a web page somewhere and include the link (as notes_url) in your service definition.