* Species Variation: There are many different species of hamsters and gerbils, each with its own size range. Some hamsters are very small, while others are quite large. The same is true for gerbils.
* Individual Differences: Just like humans, individual hamsters and gerbils can vary in size and weight based on their genetics, diet, and activity levels.
* Health and Diet: An overweight hamster or gerbil might be fatter than a healthy, lean hamster or gerbil of the same species.
In general, hamsters tend to be heavier than gerbils, but there are exceptions to this. Ultimately, it's impossible to make a blanket statement about whether one species is fatter than another.