The scale is a blunt instrument. It weighs everything that left, and it cannot tell you what kind of tissue it was. When people on a GLP-1 are scanned by DEXA rather than simply weighed, the split is more even than anyone expects: in the body-composition sub-study of the STEP 1 trial, roughly 40% of the total weight lost was lean mass rather than fat (Wilding et al., 2021, NEJM).
Lean mass isn't only muscle — it includes water, glycogen and organ tissue, and some of that reduction is a normal consequence of a body getting smaller. But muscle is a substantial part of it, and muscle is the part you feel. It's the arms that look softer than the number suggests. It's the stairs. It's the weights that used to move and now don't.
It's also the tissue that burns. Less muscle means less energy used at rest, which is one of the reasons a loss that ran smoothly for four months gets stickier in the fifth. Holding on to muscle on the way down isn't vanity. It protects the shape of the result and the pace of getting there.