They're significantly larger than a normal audio file because each track is stored separately in addition to the fully mixed song, and audiophiles will be disappointed to learn that they're not available in a lossless format.
The theory is that producers will be interested in upcharging for Stems files versus conventional audio formats, but higher price isn't the only reason that everyday listeners won't want to start using them. Only when you load a Stems file into an app that specifically supports its advanced features - Native Instruments' own Traktor DJ suite, for instance - do the individual tracks within the song become available.
That may not be Stems' most interesting trick, though: the format is completely backward compatible, so you can load a Stems song into any media player that supports MP4 and have it play like a normal song. For DJs and remixers, that makes the task of reusing key components of a source track much, much easier. The format, Stems, can store up to four individual tracks within a single mixed file, which means that different components of a song (the drums and vocals, for instance) could be made available as isolated audio. Native Instruments - best known for its DJ and music production software - is rolling out a new royalty-free audio format that could be a big deal for professional and amateur remixers alike.