Let's see -- you are screaming in at a very high closing speed, and he shoots the front of your plane, and you are surprised at the amount of damage your plane takes?
You essentially received damage equivalent to a HO attack. Your return fire was much less effective, given that your shots reached him with much less velocity on the non-critical rear of his aircraft, which also happens to be larger and more sturdy than yours.
So if you think about it, the result isn't surprising at all.