Yea, I have this happen quite often, Usually large hulled ships I make in the beginning-mid portion of the game that I keep around and don't upgrade (because they usually don't need to be). They usually end up having like 200-300 hp's or more, and since they are so old they never get shot at in a fleet (since those ships weapons are like 25 attack / 15 defense, yet after all the levels, and weapons/defense bonuses they usually come out being able to deal a descent amount of damage), and newer ships always take the brunt from their fleet attack/defense modules.
I've had small hulled ships that have reached excessive high levels, from it destroying huge stacks of fleets that just keep attacking (since the AI does no power check to see if it will win or not). I had a small hull go up to 150 or more hp's. (with no hitpoint modules added) in like 5 turns.
I think (but am not sure) that the amount of experience gained is relative to how 'current or up-to-date' the ship is. So huge hulled state-of-the-art uber ships will not gain hardly anything (Dread Lords gain very little themselves unless they beat another uber ship).
But older ships that have survived a long time, have become pretty powerful due to their level gaining. But they keep gaining a ton of experience, because they are still outdated and most likely have very very old weapons and defenses on them.