Good question…but one that has a couple of problems embedded in it.

A little arithmetic will show you why.

The most calorific food known to man is pure fat (in either solid form-lard-butter, or liquid form-oil).

Fat contains approximately 9 kcal/gram. A Lb. (pound) is exactly 453.592 grams.

Now, if we take both these factors and multiply them, (453.592 g)(9 Kcals/g) we get about 4109 Kcals. So, as fat is the most calorific food known, we will have issues in trying to find something that will weigh less than fat that will simultaneously be more calorific…there cannot, as far as we know, be anything in existence which will have this characteristic.

The 2nd issue faced is that the 3500kcal/fat-lb. rule is false, even though people have been taking it as gospel-truth for the past 50 years or so. In very-few-to-no people, will consuming 3500kcal extra cause them to gain a fat-lb.

This is due to the fact that there are energy costs and expenditures involved in processing and digesting food.

How much the fat gain (differentiated from body weight or body mass gain) will be experienced will depend on multitudinous factors like age, sex, genetics, metabolic rate, composition of food calories (even if all calories consumed were from pure fat), etc.

But what you can depend on is this: that for every lb. of fat consumed: there will always be less than 1 fat-lb. gained by an individual, any individual of whatever species of mammal, fish or insect.

So, while the initial proposition of your question never in reality can happen, the converse of it can.