The problem described may indeed be due to a spring insufficient for the weight of the car but I would check the install. If you can
a) Remove (and re-install, intact) the front wheels from your car
b) Use a tape measure of other straight edge measuring device
Try this
1) Jack up car and take off the front wheel (pick a side, either side)
2) Using your measuring tape (a flashlight will be helpful here), look behind the the wheel hub and notice where the strut goes into the wheel hub. You'll see a bolt. As shown in the following diagram, measure from the centre of where the bolt goes into the bolthole to the TOP of the BOTTOM spring perch (shown as A in the image)
That distance has to be between 185 and 215 mm (7.3 inches - 8.5 inches). If out of spec, that will need to be fixed.
3) With wheel back on, and car on a ramp, the bottom perch is moved up and down to adjust the ride height of the car. But a) it cannot move outside of the 185 and 215 mm range mentioned and b) ideally the spring should not be overly pre-compressed. This is why the range is stated. In fact, when the strut is off the car, the spring should snuggly fit between the top hat and bottom perch. This should be the starting point of the bottom perch but once the adjusting starts, watch the limits outlined by KW.
4) There is another restriction you are advised to obey. As height is adjusted, for the 1802000S model I've used as an example here, the bottom of the fender lip to the centre cap of the wheel, cannot be less than 340mm, which would be considered too low for this model of coilover. It can be greater (i.e. higher) for correct KW install but this too has limits as moving the perch upward has the effect of pre-tensioning the spring to a great degree. Check this number while the car is on the ground or ramp (i.e. the car's weight is on the spring). If less than 340mm by a couple of millimetres, not that big of a deal. Get up to 5, 6, 7, 10, 15, etc. Yeah, that's out of spec and needs to be fixed by moving the perch up. Keep in mind, springs do settle. Today's 340mm might be 338mm or less in a week. Re-check.
Coilovers should be thought of as "The minimum height is X" not "I can do whatever I like", which some people try. Aren't coilovers lovely?
Note: Over-compressing the spring (accomplished by setting the ride height closer to the max, and beyond) is a little like cutting it. Excessive pre-tensioning causes some of the 'springiness' to be used up. Ideally you want the car, not the perch, using as much spring as possible. It was mentioned by another poster that higher isn't better. I'd agree.