(    I am not a mechanical engineer (just an electrical...), however... 
My copy of the machinery handbook gives a eqn for the spring rate (lbs/in)
 of a spring like this as:

    rate = (G*d^4)/(8*N*D^3)

    The inportant items here are d and N I think. d is the diameter of 
the wire used to make the spring and N is the number of coils. If nothing 
else was different between the 2 except for the wire size, the spring with
 the bigger wire would be about 25% stiffer using your numbers. So, a small
 difference in the spring wire diameter makes a big difference! The other
 variables G and D are the material "springiness" of the steel and the OD
 of the spring. The other thing that is interesting here is if N goes down
 (cause you cut off the coil to make it shorter to lower the car) then the 
remaining spring gets stiffer. For example, say N was 6 and you cut a coil
 to make it 5. Then the spring rate increased about 20% Again, a pretty big
 difference!)


The effect on the stiffness due to changes in the diameter of the spring 
wire is to the 4th power! So, all else being equal, increasing the diameter
 from 0.66" to 0.72" is a 41% increase:

0.66^4=.1897
0.72^4=.2687

.2687/.1897=1.416 or 1.416 times as stiff, ie 41% stiffer. Spring "rate"
 means how much the spring compresses for a given load, expressed in lbs/inch.
 So, if a spring is 1.416 times as stiff as a different spring, then it will
 compress 70.6% percent as much with a given load (1/1.416).

Now, if there are more coils in the spring, since that factor works the 
other way, so more coils make for a less stiff spring (again, all else
 being equal). In practice, hard to compare 2 springs when the wire diameter
 and number of coils are different. Free length (uncompressed) comes into 
this too.

Make sure the pinch bolt on the top of the kingpin is not stuck under the
 flange on the frame. I have heard this can happen and hang up the suspension.
 If its not that or your suspension bushings being too tight/binding then
 your springs are probably too stiff.

Jeff Hopman, ND  - SDC Forum