There is a huge difference between fillings and crowns in terms of strength. What most people don't realize is that fillings actually weaken teeth, not strengthen them. The purpose of a filling is simply to fill a hole in a tooth much like spackle fills a hole in drywall with no strength. Could you imagine hanging a painting on a screw that was placed in spackle? Well, when you place in a filling that extends into a compressed functional part of the tooth, that is essentially what is happening. The filling actually draws strength from the remaining surrounding tooth structure which is why twice as much tooth as filling is required, and when fillings violate this ratio, you see cracking and breaking of the surrounding tooth structure. The result of this damage requires a more aggressive crown than the one that would have needed to be done in the first place.