Why does the belly of a muscle bulge when contracted?

Question

Why does the belly of a muscle bulge when contracted?

Answer 1

When a muscle contracts, the overall muscle length becomes shorter due to cross bridging in the sarcomeres, with the muscle belly diameter increasing in response to the shortening of the muscle.

Answer 2

The volume of the muscle cell must stay nearly constant throughout its extension and contraction because our cells are mostly water.  During contraction the diameter of the cell must increase  to maintain this constant volume.