I used to take a simplistic view of choosing a frame size. It boiled down to choosing the largest frame that gives me sufficient top-bar clearance.
On both road and MTB bikes, I can ride long distances with just this method. Yet I'm also starting to see that fit is a highly personal choice, and, for me, that personal choice may be to select a frame one size, or even two sizes smaller, than the one that gives me sufficient clearance.
It is personal in the sense that two people with exactly the same body dimensions may choose different-sized frames.
The trouble is that I am at a loss identifying why I am going to a smaller frame. The term "ergonomics" is typically used for comfort when setting up a desk, a chair, and a monitor, and for bikes as well it may be more appropriate to seek the best ergonomics when deciding whether to choose a frame smaller than the one that gives me sufficient clearance.
Can I think of bike fit purely as bike ergonomics, or is there a distinct or an additional aspect to the choice?