As light leaves a bulb it begins to spread out. The farther one gets from the bulb, the more two things happen. The light intensity is reduced and the amount of area illuminated is increased.
In a shallow tank, if the light fixture is close to the surface of the tank, then the light will hit the substrate before it has spread out enough to provide light out to the edges of this range. For this reason one must compensate in one of several ways. One is more bulbs but at fewer watts. This lets you achieve coverage (can be hard to do on small tanks except with LEDs). Another method is to use brighter lighting but to suspend it above the tank far enough to cover the entire bottom and let the distance reduce the intensity. (See the pics in the link here
http/www.practicalfishkeeping.co.uk/content.php?sid=5633 )
Very shallow or very deep tanks both need more lighting but for opposite reasons. One to achieve coverage and the other to achieve intensity.