For 12-year-old Buffy of Calabash, N.C., the trouble began with too much steak (and chicken and ice cream) at dinnertime. In nearby Ocean Isle Beach, six-year-old Hershey harbors a fondness for beef and cheese snacks. And 14-year-old Fridge of Longwood, Fla., gets cranky if his bowl isn't full.

Buffy, Hershey and Fridge are pets battling excess weight and obesity. As more Americans confront their own weight issues, furry housemates increasingly struggle alongside them. New data due out this week indicate the problem is reaching...