Gymnast try holding their bwt down to a certain limit. Easier when preforming their event. Women gymnast come to mind as the worse example of being too underweight. Both tend to be smaller framed than the average athlete.
Not always daily workouts on the bar, rings,floor, etc. Lot of days are devoted to stretches and light other exercises. Even adding weight resistance to some of their training moves/exercises. Never the less, the whole body is getting a all around pump. Which in turn offers greater muscle volume. If you were to stand next to a male gymnast you would be impressed by the muscular shape. Notable is the wider shoulders (bar and ring work help this) width. Though muscle size is somewhat limited due to their lower bwt. Stand next to a girl/women gymnast to see how really tiny they are. Good Luck.