how? less fat?
And I am not sure that it is obvious that they have the same level of fat.
My point is that muscle cells across animals don't have significantly differing density. However, different individuals (of any species) may have different levels of glycogen and fat embedded in the muscle, which may contribute to your point of "muscle density". It is just a loser terminology of muscle density as such. Even then, I doubt that muscles mass for the same volume of muscle could change that much across individuals.
All in all, I am just arguing for more specificity in your hypothesis, because I think in its current form, it isn't adding much clear detail.
https://www.quora.com/Are-male-African-lions-more-muscular-a...
This suggests tigers have 72% muscle mass vs 50% hence more density?