Spa-Francorchamps is technically a low-drag track. It's not that it doesn't require downforce, just that the most demanding corners are taken at high-speed and at those elevated speeds small wings will easily create the downforce required. The smaller wings therefore provide a drag benefit to aid top speed along the straights.
Thus we see teams bring elements of their Canadian Grand Prix packages to Belgium and also test parts ahead of the even lower drag demands of the Italian GP.
Quite how low teams can afford to go on the wing size varies. Most teams will even bring two different wing sets to compare, the drivers' confidence at these low wing levels being as important as the simulations conducted at the factory.