So - in a pound of body fat there are 3,500 calories, give or take.
Suppose that a person, all other things being equal, were to add the ingestion of 1 pound of beef tallow (about 3,600 calores) to their daily food intake, for ten days.
Gain 10 pounds, or thereabouts.
Gain some other amount of weight substantially different from 10 pounds (like 5 pounds, or 20 pounds, but not 10.5 pounds)
Or gain nothing.
Were I not vegetarian, I would try it myself and post the results. Really.