The thing with small hitboxes is that it can make the experience a bit frustrating when you see the cursor above the bug, click it and nothing happens. I think that if you wan't the hitboxes to be small, you could scale the bugs down to match them. If you think that big bugs might make it too easy, I think you could do something like bigger bugs require more clicks to kill.
Just an idea though, you will probably get comments from people that enjoy smaller hit boxes, it's just a personal preference!