Heavier wolves deal a little more damage than lighter wolves, but have a slight decrease in agility. Lighter wolves have a slight increase in agility but deal a little bit less damage than heavier wolves.
Here is a chart:
75-90 +5 Agility -5 Damage
90-110 +2 Agility -2 Damage
110-130 -2 Agility +2 Damage
130-150 -5 Agility +5 Damage