If a link is surrounded by whitespace, it makes sense for that space to be clickable (this is a common mistake). But that isn't the case on HN; the only way to make the comment link targets bigger (at least vertically) would be to add whitespace around them, thus reducing density and requiring more scrolling. This case does not seem clear-cut to me.
It could. You'd want to choose the expansion size judiciously, with less size for closely-packed links. Making the background light up during hovering would really help disambiguate click targets.
