What do Belgians eat, aside from croissants, frites, mussels, chocolates, and waffles?
The French pâtissier I used to work for in Toronto would have answered "Nothing!" His unchanging (yet frequently used) joke about the Belgians was that it was all they ate, with heavy emphasis on the moules et frites. Boyfriend graciously bore being the brunt of this joke for pretty much the entire time he was in Canada, and now that I'm here, I can say with authority that fortunately, the Frenchman was wrong.
It's not the easiest thing to explain or classify "Belgian food". Many people like to say something along the lines of "French quality, German generosity", but it's really not that simple. Firstly, quality is such a vague word. While Belgian cuisine is certainly influenced by French and German cuisine, it is definitely distinct from both. There are traces of influence from many of the country's previous occupants/invaders/rulers, including the Vikings, Romans, Spanish, English, Austrians, Dutch, Germans, and French. (More recent influences affecting "modern Belgian cuisine" include Italian and North African.)
I'd have to say that the desire to begin with ingredients of high quality would be one aspect of similarity to the French. Defining quality very simply, I can say this: It is a newsworthy scandal in Belgium when a butcher is "controlled" (inspected), and the sausages are discovered to have an incorrect percentage of fat in them, or not enough beef to warrant being called a "beef & pork" sausage. For a society where "Fîlet American" (a dish involving raw beef and often raw egg as well,) can be found readily in restaurants, butchers, and supermarket delis, ensuring the ingredients are fresh and correctly prepared is important.
Serving size really depends on where you are, however it's definitely rare to have the "tiny morsel on a vast expanse of plate compensated for by a whimsical balsamic drizzle" sort of experience here. Don't expect giant portions on account of the "generosity" reference, however. I think the "German generosity" actually comes into play when you think about the way traditional Belgian cuisine sits, and feels in your stomach. It's hard to find a meal that leaves you feeling like you weren't given enough here. Even salads normally contain goat's cheese, chicken, beef or fish making them very satisfying.
Sadly for the arteries and colons of the Belgian population, the way fats are used in cooking also has something to do with this 'satiety' response Belgian food delivers. Butter is sometimes used in frightening quantity, as is cream. At times the fat is necessary for the dish to turn out as it should (frîtes, crème fraîche chocolate truffles,) but other times you can easily reduce the amount of butter called for, or use a lighter cream with no noticeable change in the dish that results. Laying the fat on thick is more tradition than necessity with many Belgian dishes. You've been warned!