Thank you, Noa, for correcting this. Being in a hurry, I wasn't looking right.
The biblical dictionary does give a hint though: The (assumed) verbal root RVQ lies behind the term marbeq = stall, and the explanation there says, that the calves were tied up there for fattening. This makes sense to me.
Our (western) beauty ideal seems to be far a way from any comparison of a beautiful woman with a fattened calf, but that may have been different in biblical times.