The amount of text seen is dependant on your screen size... for instance you'd see more text in landscape orientation.
Typically in Android the ContentText would not expand further than 2 lines, else will end with the continuation "...". This is for several reasons; firstly backwards compatability with devices running a version prior to Android 4.1 which do not have the ability to expand text notifications, and secondly; for cleanliness. Apps using the .setContentText object of a notification will push the entire text to that object. In the case of Spotify it's the Playlist & ALL tracks which have been added (as an example). In some cases this would include hundreds of tracks and make a notification unreadable. Hence why Android limits it to just 2 lines of text.
TL;DR: It's an Android limitation for design policy.
Please use the search feature before posting a problem/bug/idea. What's a Spotify Rock Star, and how do I become one? Using Spotify Since March 10th 2009 Twitter: @mechanimal82
So shouldn't it be better to write less things into the notification bar (like only "Playlist <xyz> changed" or "Spotify alert") and when I click on the notification Android opens Spotify app and show me the full notification text in the "Inbox" (or directly in the playlist hilighting in a different color the new netries and in grey the removed songs)?
Because right now You cannot read the notification and when You click on it the notification is lost, so You never know what playlist was changed and what song was added/removed.