Springs can and do sag, but if I remember correctly from replacing my front shocks, they took a surprising amount of force to compress. This would indicate that the shocks have at least some amount of pressure in them that would make a difference to ride height. On other cars I have owned, the shocks were resistant to movement, but did not expand on their own, meaning they were 100% dampeners. Either way, its unlikely that the front springs have gotten taller with age, which means either the rear is sitting low, or the front springs have been swapped to a firmer set. If so, and if the OP can provide the chassis code for their car, or even the part number from the springs, we can help verify the correct fitment. A picture of the car or measurement of ride height at a known point might help, though I can only comapare to my diesel sedan.