PCB trace capacitance
Capacitance per unit length is proportional to trace width (neglecting edge effects).

The basic "Parallel-plate capacitor" capacitor formula for capacitance is

$$Capacitance = \epsilon * Area / DielectricThickness$$

The area of a PCB trace is the width multiplied by the length, so 

$$Capacitance = \epsilon * TraceWidth * Length / Thickness$$

Now divide Capacitance by Length:

$$CapacitancePerUnitLength = \epsilon * TraceWidth / PCBThickness$$

This is assuming trace width is much greater than PCB thickness, and neglecting edge effects, as well as the trapezoidal artifact from PCB etch. Good first order approximation, and generally good enough for most common PCB work.

If you're using an aspect ratio where the trace width is very small, and somewhat close to the thickness of the PCB dielectric layer, then you may need to go back to Maxwell's laws for the more complicated solution... but the basic principle is the same.