It's not a memory leak.  I can tell you exactly the issue and I can tell you what we're trying to do to address it.

The issue: Players and AI design their own ships. Over the course of the game, more and more ship designs get made -- not just by you but by the computer players as well.

When you load up a saved game, the game does a "garbage collection" and clears the memory of any design that is currently not being used.

The solution: To have an in-game garbage collection that goes through and finds ship designs not being used and deallocates the memory.  But it's a tricky operation to do in game and not one we want to jump on until we've gotten everyone who has any sort of stability issue taken care of.