This is usually due more in what you have coded and not how much code there is. Without seeing the actual error stack trace, it would impossible to help in fixing it.

It's saying that the heap is exhausted. The heap is used to store objects and arrays. In Greenfoot, the most common cause of this is that you have a too-large image in your scenario's images folder. Another way of causing it is by having a "memory leak" in your program, where you continuously create more objects and do not reclaim old objects. In Java programs this isn't so common, since the "garbage collection" process normally will automatically re-claim heap space from unused objects, but it is still possible. Sometimes the problem occurs simply because you are creating too many objects. As a general answer, either increase the size of the heap or create less objects.