They're not fat, they're just big-brained: Tiny spiders have such huge brains for their body sizes that the organs can spill into the animals' body cavities, a new study shows.

Such big brains may explain why very small spiders—some less than a millimeter across—are just as good at spinning webs as bigger arachnids.

For the study, a team led by Bill Eberhard, a staff scientist at the Smithsonian Tropical Research Institute and a professor at the University of Costa Rica, examined nine spider species from six web-weaving families.

The researchers found that the smaller the spider, the bigger its brain relative to its body size.

In some spiders, the central nervous system took up nearly 80 percent of the space in their bodies, sometimes even spilling into their legs.

And the brain-filled bodies of some baby spiders—such as the young of the orb-weaver Leucauge mariana—bulge until the spiders grow to adult size.

Spider Burglars Are Brainy, Too

Taking up so much body space for a brain would seem to be a problem for a spider's other organs, Eberhard said. "But [that aspect] hasn't really been studied."

Just by the way the spiders look, though, it would make sense that the arachnids are trading something for their big brains.

For instance, in the jumping spider Phidippus clarus, which the researchers examined in a separate study, the adult's digestive system is in the spider's cephalothorax—its head and body cavity.

But "in the young one, all that stuff is filled up with brain," and the baby spider has a less developed digestive system. It's still unclear, though, what impact this has on the developing spiders.

Presumably, large brains are necessary to spin webs, a behavior thought to be more complex that, say, "a larval beetle that simply eats its way through the fungus where it lives," Eberhard wrote in an article describing the research.

Still, three so-called kleptoparasitic spiders—which have lost the ability to spin webs and instead steal prey from other spiders—had "no signs of having a relatively smaller brain," he said.

Of course, he added, being sneaky and stealthy also requires a certain level of smarts, which may explain why spider burglars seem to be just as brainy as their web-weaving counterparts.