180 cm tall woman.
Weighs 80 Kg,
Looks as if she would weigh only 60 kg.
How does this work?
When comparing the weights of things it's assumed that you compare equal volume.
So muscle weighs more than fat
Since muscle weighs more than fat, if she lifts and has strong muscles she would weigh more than she looks. This is especially true if she has strong legs, since legs hide muscle well so she could have muscular legs but not even look like it if she's wearing pants