I dont think track width comes into it much, unless it's a high current track.
Boards are getting denser and it's mostly what is the smallest via I can use without incuring a manufacturing cost penalty. The maority of via's on a board will be on tracks with signal or data and low current.
We use a 40/20 thou via pretty much as standard, and maybe somthing larger for power lines.
0.5mm drill size causes no problems and most pcb fabs will handle 0.3mm without complaining. Any smaller and the costs go up!