For puppies and kittens, size really does matter.

Shelters say smaller animals get adopted faster, and animal experts say the runt of a litter tends to be better protected by the mother. Prospective pet owners, attracted to big heads on little bodies, heap attention on them.

“Humans are drawn to animals or beings of any kind whose proportion of eyes to head is large,” says Dr. Julie Meadows, a faculty veterinarian at the William R. Pritchard Veterinary Medical Teaching Hospital at the University of California, Davis. “It’s why we all coo” when we see babies, whether they’re human or animal.

For runts destined to become family pets, size is their greatest risk before birth but their greatest appeal after birth.

“It’s the underdog, under cat thing,” says Gayle Guthrie, founder-director of Stray Love Foundation in Magnolia Springs, Ala.

At Stray Love, smaller rescue dogs are adopted five times faster than the larger ones. Meadows says that could be a result of the growing popularity of so-called pocket puppies – teacup dogs bred to be small and stay small.

“Pet owners are looking for that really cute runt equivalent, almost like we are selecting for runted creatures because we like those little things that can ride around in our purses and strollers,” she says.

A litter has only one true runt, but not every litter will have one. Litter-bearing mothers have Y-shaped uteruses. Those at the center of the Y get the least amount of nourishment, while those closest to the mother’s blood supply get the most and have the highest birth weights, Meadows says.

When runts are born, “they have to fight harder because they are small, weak, and others often pick on them or push them away from their food source. All of these things tend to press on the mother in many of us to protect them,” Guthrie says.

If the runt of a litter makes it to six to eight weeks, it will probably survive and likely grow close to full size, experts says.

Cheddar, the runted kitten of an abandoned litter that Kristin Ramsdell fostered for the Black and Orange Cat Foundation, now weighs more than 7 pounds. When he was found in June 2011 with his 8-week-old littermates, he weighed less than 8 ounces — one-fourth to one-third of normal.

“I stayed up for three straight days with him, giving him fluids and antibiotics, warming him with IV bags heated in the microwave, using a humidifier and watching him round-the-clock. I didn’t think he would make it,” Ramsdell says.

He did, and was adopted with one of his siblings, Colby, by a family that reports he is thriving.

Kathy Covey of the Cat Adoption Team in Sherwood, Ore., says Little Big Burger weighed 11 ounces and had a kidney infection when he arrived in August at age 6 1/2 weeks old.

“He was on fluids, syringe feeding, pain meds and antibiotics. When you picked him up, you could feel each of his ribs. But he was a lover, snuggling into you whenever you showed any affection and purring the whole time,” she says.

Little Big Burger gained a pound in two weeks, Covey says. He has to stay on antibiotics for his kidneys but is improving. “He’s not giving up, so I’m not,” she says.

Runts aren’t welcomed everywhere, though. Wilbur, the runted pig in the classic children’s book Charlotte’s Web, was saved from slaughter with the help of a spider, but producers in real life aren’t as forgiving. A pig farmer will probably cull runts from his pens because they will never reach the body size needed for meat production, Meadows says.

The vet also notes that in the wild, only the strong survive. And runts likely won’t win sporting awards, since they won’t have the muscles or build needed for agility or show ring competition.

And animal welfare groups don’t champion all runts to families. The Cat Adoption Team wants to place as many kittens as possible, but it will draw the line with some runts, says operations manager Kristi Brooks.

“If there are a lot of rambunctious kids, we suggest that a bigger kitten might fare better,” she says.