But if there isn't anything to hold the flesh, shouldn't it stretch.
If you take your flesh into two fingers and try to pull, it stretches.
I don't think you really understand what you're observing. Your muscle is pliable so what?
When there isn't anything to hold flesh it tends to shorten. Muscle has a natural tendency to shorten. If you sit all day your calves will be relaxed all day so they end up getting shorter over time.
Your post was in response to somebody saying that spines get shorter when they're injured. Let me explain. The muscles around the area of an injury contract, and so the vertebral muscles tighten. It puts pressure on the cartilage holding the vertebrae together and it deteriorates. Chronic inflammation can also cause bone to deteriorate. Less cartilage and/or smaller vertebrae = shorter spine.