Oversizing wire however actually generates more load resistance to begin with and should not be overly done or practiced as you decrease the overall cpacity of your storage system...which is the making the batteries work harder and deplete faster...
Oversizing you have to consider also distance and gauge ensure sure you do not see more than a 3% drop in voltage over the run... (if you can make sense of it - the charts http://www.powerstream.com/Wire_Size.htm
A checklist however somewhat related:
(good site in general)
Not that it is an evil practice -however....really should be avoided...
VD= 2*K*I*L/ CSA
VD= VOLTAGE DROP
L= DISTANCE OF THE LOAD FROM THE OUTLET (IN FT)
K= 12 FOR COPPER AND 19 ALUMINUM (FOR MAXIMUM TEMP OF 75 DC)
CSA= CONDUCTOR CROSS SECTIONAL AREA IN CIRCULAR MILS.