Ah, yes, that's another thing to note too! ENFJs are not always bubbly! Nope, they can be depressed too, then they become cynical and withdrawn (turn to excessive Ti). Thus describing the line between ENFJ and INFJ with bubbliness doesn't seem precise enough.
I'll even go so far to claim that INFJs are more bubbly than ENFJs on average, because ENFJs are more stable, more calm; their primary function is a judging function. INFJ's primary function is a perceiving function which makes them a ball of passion.