Rarely will you find a Border Collie overweight. Generally they exercise so much that weight gain is usually not an issue.
Most Border Collies will appear skinny and this is actually the way a healthy Border Collie should look like. Thin through the hips is ideal as extra weight puts undue stress on the joints. Though Border Collie sizes vary somewhat as some are bigger then others, most Border Collies should weight around 40 pounds or less.