For instance, at some point it is a disadvantage to be TOO big. Natural and sexual selection are very similar in their inner working. Both require variation in the population, both require heritability of the traits, and both require some reproductive differences associated with trait differences. They are different in that sexual selection generally causes on sex to evolve differently from the other. For instance, male sea lions are a lot larger than female sea lions.
Body size is said to be sexually dimorphic.
Natural selection typically acts on the population or species in the same way.