The rig tension is basically a function of the force that the rig can generate and the geometry of the shrouds/forestay. The higher up the mast and the farther out the shrouds anchor, the lower the rig tension will be.

Consider an 18ft skiff- basically it generates more force from the rig and the shrouds anchor more inboard as compared to an 18ft beach cat. Suppose the shrouds anchor at 3 or 3-1/2ft out from the mast base- a cat anchors at 4 or 4.25ft. The cat as two guys with 8ft or righting moment. The 18ft skiff has 3 guys on a rack 9 or 10ft from center line, so more righting moment= more rig force so more rig tension.
I believe the skiff uses a smaller mast section and they keep it from deflecting to leeward by locking it with spreaders on the lower shrouds. This requires that the leeward shroud be tensioned as well as the windward shroud.
I'm pretty sure the 18s have hydraulic rig tensioning devices.