Quick answer: For small dogs, look for a GPS tracker under 30g — most standard trackers are too heavy or bulky for small breeds and can cause collar discomfort or posture issues.
Dogs under 10 lbs should carry no more than 25g on their collar total. Dogs 10-20 lbs can manage up to 30g comfortably. Measure your dog's collar width before buying — small dog collars are usually 3/8 to 5/8 inch wide, and many tracker clips only fit wider collars.
Trackers designed for cats are often the best fit for very small dogs — they're built to the same weight constraints. Check that the collar attachment clip fits your specific collar width. Many cat-specific trackers come with adjustable straps that work well on small dog collars.
Lighter trackers have smaller batteries. Expect 1-3 days of battery life for the lightest models versus 5-7 days for standard dog trackers. If your small dog is an escape artist, charging every 2-3 days is a reasonable trade-off for a safe collar weight.
Don't put a tracker designed for a Labrador on a Chihuahua. Look specifically for trackers labeled for small dogs or cats to stay within a safe weight range.