I noticed in the mega eagle schematic that certain wire "paths" such as the gnd and 5v are thicker than the usual signal wires... Is there a genuine reason for this?
Enjoy
It reduces the impedance of the lines making the voltage drop lower between
the power input and the devices that draw high currents. It also reduces the temperature
rise of the traces and possibly some components (like the linear regulator which may
be mounted on the ground trace).
Although nothing on these uC boards draws large DC currents there are large AC
currents due to clock switching. With a high impedance trace you will see larger voltage
spikes on the Vcc lines (and possibly the AVcc if they are connected).
(* jcl *)
P.S. There is a green reason to leave more copper on the board -- less energy consumed
to produce the board and less waste 
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