Light riders are at an advantage on climbs IRL because "a light rider doesn't need to generate as much power as his heavier competitor because he has less weight to propel up hills." [1]
That's why I said "almost always". It's not like any branch of bicycle racing has weight classes. You always compete with people in your category no matter how much power they generate or how much they weigh.
Things are different downhill [2]
[1] https://www.cptips.com/climb.htm
[2] https://road.cc/content/feature/does-heavier-cyclist-descend...