I'm currently 180lbs and trying to put on more muscle mass. I'm still pretty lean, with ab veins and shoulder striations.
I've only ever been out of the overweight category (<174) once in my life. I felt sapped of energy and looked gaunt, with veins popping everywhere.
That is a super interesting observation about inseam length mattering. I had never considered that and I have always looked with amazement at people my height who seem muscular yet weigh in the 150s. I always thought I was doing something wrong.
Bikes are sized based on the horizontal length of the frame, via measurements "effective top tube length" or the similar "reach", with almost everything else being adjustable. So long as that length is good for your torso length, the other parts can be adjusted or swapped to dial in the fit. (This is why bicycle sizes tend to indicate a height range.)
The whole long torso length thing means that I tend to ride a larger frame with the seat adjusted shorter than someone with a shorter torso would. Thus, I generally ride a large from most companies, but have to be careful that the seat can be adjusted in the right position. Similar height buddies will often ride medium frames with the seat much higher. Their longer legs exacerbate the need for a shorter (length) frame because moving the seat up moves the hips rearward on the bike, making something even shorter needed. For a lot of the typically-athletic-build women (long legs) this can make it a real problem for them to find bikes as it's often difficult to have the seat high enough for their legs but a short enough reach.
(That old thing about sizing bikes based on being able to stand over them is completely wrong. Bike sizing needs to be done based on riding the bike, not standing over it.)
If my weight gets lower, into the low-to-mid-160s, even notoriously-skinny cyclist friends start to think I'm looking exceptionally thin and should watch it.
At my physicals the software automatically classifies me as overweight, but the doctor never mentions it, because he clearly gets how these categories can go wrong for folks like us. Someone my same height, with longer legs and a shorter torso, ends up 10-20 pounds just by nature of having less core. But of course BMI is just height vs. weight.