The French not only expected the Germans to come through Belgium, they WANTED them to come that way. Half the point of the Maginot Line was to force the Germans to attack via Belgium. The bulk of the French army, their best divisions, were positioned near Belgium, and as soon as the Germans invaded, the French and the British moved into Belgium to meet them.

The problem is that while Belgium is small country, it isn't that small. The southern part of Belgium has very hilly, wooded area with very narrow roads - area known as the Ardennes Forrest. Because of the geography of that particular area, the French concluded that the Germans couldn't come through there, with all their tanks and heavy equipment. Most German generals, when they were planning the invasion of France, had the same opinion. An invasion through the Ardennes would be very risky since the entire column would get bottle necked in very narrow roads. Thus the French didn't allocate adequate defense of that area. They thought that the German invasion of central Belgium was the real thing when in fact was just a diversion.

The German high command decided to take a gamble and attack through the Ardennes anyway. Their air superiority made it slightly less risky then it was assumed, since the Luftwaffe could provide cover. The gamble paid off, and the German units appeared behind the bulk for the French and British troops which were in central and northern Belgium at the time.