It wouldn't be possible in EU4. I think that it should be, though, just rare. The current battle system (especially in CK2) doesn't represent accurately at all just how uncertain battles were. This is why most wars in almost all of history were fought with sieges. Alexander the great throughout his many campaigns fought only a handful of battles compared to hundreds, possibly a thousand sieges.
EDIT: on second thought, reading some of the comments, maybe the battle was something more like 2,000 vs 20,000 and the 20,000 stack was completely low on morale - a small 100% morale stack wiping a large 0% morale stack definitely can happen in eu4