And then get a storm of downvotes that cancels out the benefit they hoped to gain by padding their comment. And then probably not do it again.

What I'd be more worried about is that short comments may be more valuable than you would think from their average karma -- e.g., perhaps in some cases short not-exceptionally-insightful comments form (as it were) the skeleton of a discussion, within which insights might emerge. Or perhaps if everyone felt they mustn't post short comments unless they were exceptionally insightful, the barrier to participation would feel high enough that scarcely anyone would ever post anything, and LW would just wither.