Healthy vs Concerning Weight

A note on thinning:

If your frog is too thin, you'll need to address the underlying problem and supportive care will be needed if the frog is emaciated. Sometimes it's simply a matter of them not eating enough, so boosting their food qty can be all that is needed. Old or improper supplements are associated with weight loss and should be checked/replaced with fresh.

Other times the issue isn't food or supplements but rather stress. A new frog can be too shy/stressed to eat properly. Unfortunately, it's seen that some businesses sell frogs that are too young and don't handle the changes well. Froglets should be well established on land and be fat when placed into new homes to give the frogs and the new keepers the best chance of success.

If the thinning frog is in a group of others, tank mate stress can weigh on a frog even if the keeper doesn't see any fighting. Not all frog stress is tough-and-tumble fighting that is common with Dendrobates tinctorius or Oophaga pumilio. One "greater" frog simply having a strong presence and eating faster/stealing food from the "lesser" frog can be enough to slowly cause the lesser frog to reduce eating. Putting the lesser frog on a vacation by separating them and putting into a quiet quarantine (QT) tank or bin can help. 

Vitamin deficiencies often go hand-in-hand with weight loss and especially with emaciation. You'll want to check brands and freshness and offer dusted feeders. It's often recommended to offer vitamin soaks in petri dishes with and without hides to allow the frog to address its needs in a low-stress environment. See Weight Loss / Emaciation for a walk-through on how to do this.

A note on chubby frogs:

While our cultures tend towards thinking small chubby animals are cute, there is significant health problems with frogs being obese or chronically chubby. Extra weight requires internal organs to work harder, the immune system is weaker, and there is more stress on bones and ligaments. 

Thankfully it's a simple fix for the animal, with the hardest part being the re-learning for the keeper. Reducing the qty or changing the type of feeders given is often all that is needed. If most of the feeders offered are very fatty such as wax worms, fruit fly maggots, or termites, then halting those feeders and feeding a different base food such as fruit flies, pinhead crickets, or firebrats can get them back to proper weight.