There are multiple causes of abscess occuring in various location in animals. I have seen them in Ceratophrys that were kept on foam rubber pads. The frogs were abrading the toe tips trying to burrow into the pads and the abrasions were allowing infections to set up and abscess.
Healthy tissue does not typically form abscess on its own. There needs to be an additional factor for the abscess to form, foreign body, necrosis, or infection at the site of the abscess.
Fat bodies are not typically deposited in anurans in the toes. Stored fat is typically stored located in the abdominal cavity as a pad between the ventral body wall and the organs.
Lipid deposits elsewhere in the body may need to be considered as a symptom of a problem with the frog.
As for mice as long as the mice are not offered too often they are nutritionally fine. The caloric needs of the frog needs to be taken into consideration so overfeeding does not occur.
This is a comparision from a post further down.
Actually house crickets contain 54% fat/kcal*, while commercial crickets contain 44% fat/kcal*, compared to 60% fat/kcal* for mealworms. Compared to a 1.5 gram pink which is 40% fat/kcal and an adult mouse (27 gram) which is 47% fat/kcal. Pinks do not take a jump in fat content until they are larger than 4 grams at which time they hit 60% fat/kcal which is no higher than mealworms.
The only low fat invertebrate that is commercially available are earthworms but a diet os solely earthworms has been linked to muscle diseases in anurans (see Modzelewski, E.H.; Culley, D.D. Jr.; 1974, Growth responses of the bull frog, Rana catesbiana, fed various live foods; Herpetologica, 30(4): 396-405)
* Donoghue, Susan; Langenberg, Julie, 1996, Nutrition, In Reptile Medicine and Surgery, edited by Mader, Douglas R., W.B. Saunders Company, Philadelphia
Ed