I noticed some aircrafts sink just slightly into the tarmac.. is that solely because of the contact points defined or can it be caused by the scenery as well? assume I'm using a properly made scenery that has solid ground.

Contact points are extremely sensitive to weight and airspeed in FSX. There is no perfect contact point setting. If the tyres look just about right when an aircraft is lightly loaded, they will tend to be sunk into the ground when fully loaded. Conversely if they look ok when fully loaded they will be barely touching the runway lightly loaded. Approaching take off speeds or having just landed, tyres can appear not to be touching the runway at all. You can alleviate this by hardening the suspension so that with any load the tyres appear roughly at the same height in relation to the surface but that has side effects that can be a problem.

 

This is all the result of FSX over-reacting to weight and speed over ground, but is also the result of there being no simulation of tyre pressures or compression. It is not (so far) possible to simulate tyre compression, only suspension strut compression.Thus most designers build in the illusion of tyre compression by placing the wheels slightly below the surface, which give the impression that the tyre has flattened into a small, uncurved section where it meets the tarmac.

Guess I should just leave it then. I hope they plan to improve this for p3d 2.0.