Sage Line 100 is, according to someone in the know, a "screen-scrape" version of the DOS Line 100. Sage MMS is an all new rewrite (though the helpfile still calls it Line 100). Despite it being based on .NET the backend is still a (shock, horror) file database. No relational database engine, just files on a network drive.
The good news is that v3 will be based on SQL server. The system is full of functionality to save space - transactions can be purged or summarised - which I've not come across. Financial data isn't that huge, but if you're using file based methods to index it then I'm sure it must matter how big your data is getting.