On your question on how it applies to larger bodyparts:
As a rough principle, first, I’d think it’s a matter of proportion. Roughly speaking the chest may be nearly 3 times the circumference of the arms. So it makes sense that it would reasonably take 2 or more added inches on the chest to seem “as much” a change in a bodypart as is the case for 1 inch on the arms.
But ultimately it is more complex than any simple method can account for.
The eye is struck by both proportions that it rarely sees, and by absolute sizes that it rarely sees.
So if adding an inch to for example the arms doesn’t accomplish, in the individual’s case, EITHER of these things, the eye may not pick it up as that big a deal.
But then a transition point will be reached where the gain does make the difference between “pretty good but seen that lots of times” to “Whoah, that’s striking.” Or otherwise moves the person up to being in a different category, so to speak, than previously.
Then there’s proportion. An extra inch on the chest – regardless of the chest being proportionally much bigger than the arms, and so one might think that an inch here would be far less meaningful – combined with an inch reduction in the waist could result in the drop and V-taper moving from ho-hum, see-that-every-day to something that grabs the attention.
Not as a night-and-day difference, of course, but significant.