Here's why:

* Size and Volume: The volume of a multicellular organism increases much faster than its surface area. This means that larger organisms need more cells to maintain a suitable surface area-to-volume ratio for efficient nutrient uptake and waste removal.

* Complexity: Larger organisms are often more complex, with specialized tissues and organs that require a larger number of cells.

* Growth: Larger organisms grow by increasing the number of cells they have. This is why a human embryo starts with a single cell and eventually develops into a trillions-of-cells-strong individual.

Exceptions:

* Density: Some organisms may have more cells packed into a smaller space, such as in highly dense tissues like bones or certain organs.

* Cellular Size: While not as common, some organisms may have larger individual cells, which could theoretically compensate for fewer cells overall.

* Organism Type: In some cases, the number of cells might not be directly proportional to size. For example, certain plants might have a high number of cells despite a relatively small size due to their structure and cellular specialization.

In conclusion: While there are exceptions, the general rule is that larger organisms tend to have more cells than smaller organisms due to the need for a larger surface area, increased complexity, and the process of growth.