Nice!
Welcome to the forums
Maybe you could use sandpaper to make a small flat spot on the tires where they contact the "ground" to give an impression of weight?
Look at a real truck or car, even if the tire doesn't bulge sideways there is still a small flat area where it rests on the ground. On a normal car with normal load and tire pressure the flat spot has the size of a palm or maybe a little more (depends on the size of your hands

).
On a truck it will be larger, maybe two hands, palms + fingers.
/ Robin