I'm totally OK with a character doing more damage on a Defend but taking damage in the process. I think it's also worth pointing out that the holds gained from the Defend move don't stack. If you have 1 hold left from a previous Defend and then roll a 10+, I'm pretty sure you *don't* have 4 hold but still only have 3 hold. The Defend move says "you hold 3" and not "you hold +3".
As a contrary opinion, I think they do stack. It does say "hold 3" not "choose 3" (as in Discern Realities). I also don't require the defended target to be "about to take damage right now", just "under attack" broadly. ("The orc army begins its charge, what do you do?" "I stand in defense of the Wizard!" "Great, roll+Con"...)
Of course, if a player just spends the whole fight defending, they'd better find some way of making that interesting, for their own sake, if nothing else!