That's not at all unexpected and inline with others' observations who have done the same.
The design characteristics between the two brands/lines are extremely different. The Cooper Cobra (and MasterCraft GT / Hercules) line were originally designed for use on full size American muscle cars with leaf spring suspensions, whereas the Falken Ziex line is a far more refined tire and actually designed for use on independent coil spring / shocks / MacPherson strut suspensions.
Most who drop from a 235 to a 225 in the rear will notice no difference, other than those of visual aesthetics, because of the way that most actually drive their cars.
Glad you're happy with your new Falken Ziex - they're a very nice tire at a very nice price.