This relationship lets us fold up compact

I find a helpful way to remember this particular proportion is to consider how it allows us to fold ourselves up into a squat position.

I like to think of this as similar to a three-piece foldaway table. Just like with the table, the three sections of the body need to be a similar size to allow for us to take this efficient, more compact position. So next time you are unsure about the proportions of the legs, consider whether your figure folds up neatly into this shape.