Naturally it shouldn't take 6gbs from start. It may keep building up eventually though. The solution is to use Save/Load option, it helps dealing with memory issues in case game starts hogging your RAM too much.
We're always looking for ways to optimise the game for better performance. However, WFTO is quite a complicated game and extremely unusual in terms of modern titles as an RTS with an ever-changing environment it can be very heavy on RAM and CPU usage especially on large and complicated custom. It is for this reason we recommend at least 8GB of RAM.