There are rates for planting seeds and for broadcasting. Each are based on the number of plants per acre after germination. Some can be seeded at higher rates than others. One condition where overpopulating would work is with high deer density or smaller plot which will get hammered. The logic behind this is the deer will consume part of growing plants allowing the remainder to grow.
You have to consider the size of the plant at maturity and not while they are seedlings.
As posted earlier, over population, in majority of the cases, will cause competition between plants. This can mean crops dying or being stunted and not reaching their potential.