to much fat in to short of time can over work the organs- slow and steady with a healthy diet wins the race.
As you can see in my pics above-Cedro arrived very underweight- he gained nearly 10 lbs the first 2 years we had him.
proper heat gradients (hot spot to cool spot) and good UVB are key when it comes to digestion and making good use of the food they eat. Without it they do not break down food or absorb the nutrients in it. Be sure youve got you set up done right and that your feeding a well rounded diet and you will see changes in no time.
If your doing all that and still your igs looking thin, have a fecal sample checked at the vet for parasites.