There's difference between ogre, ent, mumakil etc. and fellbeasts and eagles.
Ents and co. are more massive, heavier and stronger. Eagles for example have "empty bones", that's why they are so light. So, imagine now this fight Gvaihir vs. average stone wall.
All interesting things, but this is not a scientific essay, we are talking about a game, set in a fictional and fantastic World
By the way, Giant Eagles can't be compared to normal birds, in Tolkien's lore; they are magical and sentient (they think and 'speak' like Elves and Humans) creatures of Manwë, and only to Manwë obey; I think that their strength and resistance can be compared for instance to the ones of the Balrogs, or are quite similar.
In the First Age, Thorondor, the King of the Great Eagles, was able to permanently wound Morgoth in the face and 'save' the dead body of Fingolfin; the Eagles of Manwë fought also against the terrible Winged Dragons in the War of Wrath.
Not so common qualities, I guess