You have to check your sag. Get the bike up on a stand and measure the distance from the rear axle to the fender (about straight up, doesn't really matter as long as all measurements are from the same spots).
Then put the bike on the ground and measure the same distance. The difference between these measurements should be about 20-35 mm. (about an inch).
Now sit on the bike or get some support and stand on the pegs and have someone do the same measurement. This should come out to about 90-110 mm difference from the bike-on-stand measurement.
If both of the differences are off to the same direction (too much or too little) you can adjust preload to get them into range. For a spring that is too heavy, if the first difference (stand to bike on ground) was ok, the second (stand to you on bike) would be too small (less than 90mm).