That's a feature IMO -- HN could even make them even smaller.

It should take conscious effort to vote, and the small target size mentally reinforces this model; that it's not supposed to be the most common action.

EDIT: See, clearly the vote targets simply aren't small enough! </s>

I rarely wish to click on the username or time links (“3 hours ago”), but that’s what I click on the most due to size and placement issues (when trying to vote or collapse a thread). I agree one should not blindly vote, but at the same time if you have decided to vote, it shouldn’t be maddeningly difficult and error prone to execute.

Usability-wise, isn't it kind of weird that the click target you use the most (upvote arrow) is the smallest of the three? And the ones that you use rarely or never, are so easy to hit by accident. Why must world be like this?

I’ve also down voted comments by mistake...

They have added an undown link a while back.

Adding undo changed the feel of downvotes a little. Irreversible accidental downvotes were scattered randomly on comments and they made the occasional downvote feel less harsh.

But the undown/unvote links are hard to tap on mobile as well. On firefox on android it's always an exercise in patience.