The rear is a solid axle design with straight up and down coil springs, so a 2" spacer gives you 2" of rear lift. The front is IFS with the coil spring cup roughly half-way between the wheel and the A-arm's pivot. When you add a 1.5" coil spacer up front, it moves the wheel further than 1.5" because of the lever/fulcrum effect. A 1.5" spacer gives roughly 2-1/4" of front lift. Most Trackers and sidekicks have saggy front springs that make the truck sit slightly nose-down stock, so the extra 1/4" really helps level the truck and also make up for a heavier bumper or skidplate.
Think of it this way: hold a pencil in front of you by the eraser. Without letting the eraser end move, push the center of the pencil down half an inch. The tip moved much further than half an inch, right? Same thing happens with the front A-arms on the IFS.