At base, Nunu starts with 213 mana with a 42 per level growth. Regen is 6.6 with an aditional .5 per level.
His mana costs are 60 for Consume, 50 for BB, and 65+10*rank for Snowball. Slow on Snowball lasts 4s.
Shyvana has absolutely no costs on anything. You're looking at maybe four snowballs if you're not even using Consume / BB early on, and Shyvana can heavily deny Nunu last hits by superpushing into the tower so he can't easily buy more sustain. Then she can go and take doublegolems / wolves while he shoves your creeps back off of the tower.
Nunu's Snowball starts out as a 20% slow. Shyvana's W is a 30% increase at the start, and she also starts out with 10 more base movespeed than Nunu.
Maybe I'm wrong in this analysis (I've NEVER seen Nunu top), but Shyvana's ability to push will hurt him HARD early and once she gets some MR going his harass will mean literally nothing, and she can easily prevent him from farming.