Here’s the deal:
*Increased muscle cross sectional area has an extremely high correlation with force production.
*When something is rotating about an axis, it will rotate faster if its mass is distributed closer to the axis. In the case of the lower leg, this means that it is advantageous to have a high gastroc muscle attachment because it will mean that regardless of whether the muscle itself is big or small, the distribution of the muscle’s mass will be close to the axis of rotation (the knee). If you’re having trouble understanding this concept, pick up a wooden baseball bat. First swing it by the handle. Then turn it backwards so that you’re holding the wrong (heavy) end. You’ll soon find out how much easier it is to swing the bat fast when the bulk of the bass is located close to the axis of rotation.
*Increased tendon length should increase the elasticity of the musculotindinous unit.
Having said all that, there are obviously tradeoffs. A really big gastroc muscle should in theory be capable of producing more force. It will however require more force be produced from the upper thigh and hip muscles to move the bigger and heavier leg. The ideal lower leg musculature seems to be a heavily muscled, medium sized gastroc that has a high muscle attachment and a long achilles tendon.