I've heard it said that the force a muscle can exert is proportional to its cross-sectional area. It seems to me it should be proportional to length also. If you have someone pulling on a large block, you could increase the force by getting another person to pull in parallel, OR by attaching a rope to the block and having both of them pull in series on that one rope. So shouldn't muscle strength be proportional to both area and length, making it proportional to volume?
Does the answer have to do with the fact that material strength is proportional to cross-sectional area, so this puts a limit on the amount of force that muscles can exert without breaking?