If the footprint is larger than the tent, during rain (or heavy dew), water runs down the tent wall and collects on the waterproof footprint. If you're on a soft surface or grass, your body weight will depress the tent floor and footprint, putting the lowest point in the center. Your tent is now sitting in the middle of an artificial lake. Any pinholes in the fabric of the tent floor will allow water to flow in.

A smaller footprint allows the tent wall to funnel the water onto the ground. The waterproof footprint should prevent water from the saturated ground from reaching the tent floor from below.

The last tent I purchased had a ready-made footprint was about 1" smaller all around, attached to the corner poles with tiny resin clips on wee, tiny straps. Before that I just used cheap blue tarp cut slightly smaller than the tent floor.

One drawback to the pre-cut footprints that remain attached is stuff can get caught in between the footprint and the floor. Roll over at night and squanch a mouse and it'll stick there. What's that smell?