[AgentZero]

Well, it would be relative. If each hull size has an experience 'value' and you get a bit extra on top of that value by destroying one ship with a smaller one, and a bit less if a larger ship takes out a smaller one. So, to answer your question, lets say each 100kT of ship gives 10 experience points. and you get 10% plus or minus for each 100kT difference between the hull types.
Therefore, it the case of your 1000kT vs 600kT, the ship destroyed has an experience value of 6000, but the victor is penalized 2400 pts (40% of 6000 since there's 400kT difference between the two) so it gets 3600 experience.

In the case of the 300kT ship taking out the 500kT, the destroyed ship has an experience value of 5000, plus a 20% bonus for the difference in ship size=6000. Which makes sense, because it'd be a lot easier for a 1000kT ship to take out a 600kT ship than for a 300kT to take out a 500kT.