Do stallions have an easier time building muscle than geldings?

I spend way too much time looking at horse ads and one thing I noticed was that stallions almost always seem to have a better topline than geldings. I have three theories

Only the horses with the best toplines are allowed to remain intacf

Stallions are more active because of typical stallion behavior and it means they are building more muscle

Malw horses benefit from testosterone to fuel the chemical muscle building mechanisms

Some combination of the three?