I did not know that yesterday!

I start by noting that he crucial difference between ketchup and mustard bottles isn't so much the shape of the container as the size of the mouth. After years of scientific study, condiment researchers discovered that whereas most people delicately daub mustard on with a knife, ketchup they pretty much slobber over everything straight out of the bottle. That's because mustard is a pungent substance that will burn holes in the roof of your mouth if used to excess, thereby giving us the thrill of living on the edge of danger that is so lacking in contemporary life. Ketchup, in contrast, is bland. The bottle mouths are designed to accommodate the differences in usage.

Interestingly, the H.J. Heinz company did put a wide-mouthed ketchup bottle on the market in 1966, for people who wanted to use ketchup by the spoonful in recipes. This daring innovation promptly bombed monstrously (although it is still sold in a few places), apparently because Heinz had overlooked the fact that for the most part there aren't any recipes that call for ketchup, except as a glaze--most people use tomato sauce instead. In contrast, quite a few recipes for sauces and dressings and whatnot require mustard. At any rate, French's does make a mustard bottle with a narrow mouth for use in restaurants, where you want to discourage people from poking their grubby cutlery into the condiments.