Genes play a huge role in individual variability of outcomes, whether it's leanness or obesity, strength, cardio/fitness, academic ability etc.
An example I like to give is, my junior high school had an annual writing competition. The winners were so far ahead in terms of ability ,which was evident when they read their stories aloud , that such ability had to have been divined in some way. There is no way you can be competitive when some have such a huge head start due to genes.