Well, that isn't what people mean when they ask how heavy was my biggest client (several in the 300 to 390 range, thank you.) And here is the real basic answer, which is bones!
Every body has bones. Each muscle anchors to a bone. (Your thigh muscles attach below the knee and up at the hip, for instance.) All I ever need to do is locate the bony landmarks. The muscles, tendons and ligaments unfold for me from there. Now I am in my familiar world of texture, the feel of the tissues, and can still tell what is taut, sore, overstretched etc. My table is adjustable as we work and my work posture is still comfortable and strong.
I have found it is prudent to modify the amount of weight-bearing stretches and supporting moves I use if someone is over about 275 pounds. An example is assisted leg stretches, especially hamstrings. My own frame needs respect!
Still, I never judge weight. I do applaud healthy mobility. For many clients I find good digestion and elimination can improve memory and sleep. I find that my own best weight isn't my highest. I'm just matter-of-fact about weight!