-- Last Updated: Dec-20-12 9:46 AM EST --
The Zephyr does not need to be muscled around unless the paddler fails to put it on edge. That sentence alone says the issue was about your unwillingness to go on edge rather than the boat. Even if you are undersized for the Zephyr you had, once you haul your butt into the bilge to get it on edge the sense of having to muscle it around turns should diminish.
It is possible that the sea kayaks you have tried before included some number that were heavy duty trackers and/or big on you. If you are not comfortable on edge, and/or do not understand how a boat works that way, you would interpret the boat going over onto its edge as "tippiness" rather than understanding it is just settling onto a second quite stable point.
A more maneuverable boat may drop onto its edge more readily than a heavy duty tracker, for reasons ranging from the conditions to the paddler's own technique (or lack thereof). Remember that you are likely moving around and doing corrections more in a boat that is quite maneuverable - all of that will cause a reaction in the hull.
This is a normal issue for people coming from wide bargey boats into sea kayaks, especially one that likes to turn like the Zephyr. It just takes time, and often work on paddling strokes, to get around it.