It's not the pinion that is weaker it, its the stress exerted onto the teeth that is higher. With a numaricly higher gear ratio you have less pinion teeth, there for less contact withthe ring gear. When there is less pinion contact with the ring gear the force exerted onto the pinion teeth rises and causes the failure. The more teeth in contact the more the load is spread
Just like a wide tire vs nerrow. The nerrow tire has more pressure per square inch than the wide one because of the size of the contact patch.
This is a verry imformative post right here /\
I too belive the strength differences are minimal. The ring gear tooth strength is what conscernes me with going with lower(higher numerical) gearing. The added amount of teeth on the reing gear means the teeth have to get smaller (thinner) as the tooth count goes up. Add to that the added stress, and lower contact patch and a heavy foot and you get this.
