My pet damage differential during stampede is definitely due to the number of hits they do.
Thundermaw - 10+ hits on every stampede
Thornfang Ravager - 2-3 hits on most stampede's with a couple around 4-5. When this pet is my main pet it doesn't seem to lose attacks, its only when used with stampede.
I have also used the same pet as Trictagon - Shimmerscale
The Shimmerscale pet with Rabid does 27 attacks, none of my other pets do this with Rabid on a regular belt. Rabid is up for every belt.
Anytime I have used this pet with Stampede + Glyph it has done 65-70 (~13+ attacks per pet), which is what you should expect excluding Bloodlust/Heroism.