The number of producers is much lower than those of the primary consumers. How can this be

This can happen if the producer is large in size, or the primary consumers are small in size.

Example is where the producer of a food chain is a type of tree, and the primary producer is a type of insect. Clearly, an insect cannot consume the whole tree by itself, therefore, one tree can provide energy for a lot of insects, thus the number of producer is much lower than those of the primary consumers.