Hey...
at 6'1 and 120 (not for a long time, gratefully)... I was a size 9.
at 6'1 and 170 (not for a long time :P) ... I was also a size 9
Just the size of my bones.
PART of why "no one believes" us about weight is that women lie, and men aren't as stupid as we make them out to be and go deaf about weight rather early in life.
PART of why 'no one believes us' is height. When someone is significantly shorter or taller than you... weight distributes differently. Myself, for example, I run 30lbs per SIZE. So if I want to go from a 9/10 to an 11/12 I have to go 30 pounds. One of my besties is 12 inches shorter than me, and she's 5lbs per size. Most people are around 10lbs per size. ((I couldn't figure out for the loooooongest time why on earth people cared about 'the last 10lbs'. Not until I realized that was a whole size for them)).
PART is bone density and shape. I'm currently fat, but I really am 'big boned'. :P I have girlfriends who, being the same height as me, have delicate bones and fit into 6's and 4's at 120lbs. Bone structure and density affects both weight and size.
PART is boobilage.
PART is skin (I have 30lbs of skin I'd like to say buhbye to myself) if you've lost a lot of weight, or it doesn't rebound after baby).
PART is confidence (act confidant / sexy and people see you that way... which either adds or subtracts pounds depending on your culture. Which is a strange thing... our eyes lie to us).
PART is distribution / how the weight is carried
PART is bonestructure, again, but this time in the distribution. Some people are TINY but have round faces so 'look' heavier than someone who weighs waaaaaaay more. Other people do the jowly thing, or the goddess cheekbone thing, or, or, or, or.
PART is skin type (darker skin = thinner appearance / tighter skin = smaller size, looser skin = larger size)
PART is clothing style/type.
<grin> Sorry... you combined 2 of my favorite things; art and physiology.