Why does SignalTap II Logic analyzer set the Memory automatically?

If I create a SignalTap logic analyzer file using "New-> SignalTap logic analyzer file" instead of using Megawizard, then it will create an empty instance then I can add the pins or regs I want to watch.

After I add the pins, the the costed LE and Memory will be showed (see attachment). I don't know how it calculate the memory size needed, and why does it set it automatically?

And If want to change the memory size, how should I do?

Thanks very much.