It's just math. If you are too lean and muscular, your density is much higher than if you have a higher body fat composition, but there's also an element for no

It's just math. If you are too lean and muscular, your density is much higher than if you have a higher body fat composition, but there's also an element for non-fat people and adults that they need to 1) hold their lungs more full than normal with diaphragmatic breathing, holding the breath in for longer, and exhaling more shallowly than a full exhale that they can apply, as well as 2) leaning back much farther and tilting your head back much more than you'd expect to have to. Legs sinking is a classic problem for swimmers that occurs due to body alignment issues and especially not leaning your head and neck far enough back. You need to be an outlier to not be able to overcome a body composition / density problem with techniques #1 and #2 above though, without observing you no one can really say if you're applying those techniques properly.