The Mass of Calories

Observation:
It’s common practice by bodybuilders to use high calorie diets in order to gain weight, the idea is that an excess of caloric energy would lead to additional energy stores and furthermore increased weight gain. Is this weight gain actually a product of eating higher quantities of dense foods or is there a relationship between calories and density? There are several confounding factors that make this an exceptionally difficult question to answer, the most pressing being: what low calorie foods can we compare with high calorie foods that have similar ingredients?
Hypothesis:
High density foods have higher calories than low density foods, meaning that it may be possible to give a physical weight to caloric energy.
Experiment 1:
Using a precise milligram scale and a 2 millileter tray several different liquids of different caloric amounts were weighed. The main two liquids of interest being Pepsi versus Diet Pepsi, which share all of the same ingredients except for High Fructose Corn Syrup/Sugar and Potassium Benzoate/Aspartame respectively. To further examine the density and calories these two ingredients might affect , 100% High Fructose Corn Syrup was also weighed. The results are in the table below
Liquid
Calories per tbsp
Density (g per 2 ml)
Water
0
2.1
Diet Pepsi
0
2.3
Pepsi
6.25
2.7
High Fructose Corn Syrup
57
2.6
Milk
9
2.2

Observation:
Though there is certainly an interesting correlation between the calories and density between the first four liquids which might suggest the hypothesis is correct, our fifth liquid, milk is lower density than zero calorie Diet Pepsi. However this is also homogenized skim milk which contains roughly 90% water, which with its increased density over