I just used like 13 pieces of Boar meat in 1 Dead Mines run… hes lv6 Loyalty. What is going on today?
The lower the level of food compared to your pet, the more it takes. You farm level one boars and get meat then it will take a lot to make them happy.
Retailer.
As Dottie said ^^
Just go find a meat vendor and buy a stack of meat (or whatever your pet can eat) that is as close to it’s level as possible - makes life managing things a lot easier! 
I’ve been playing hunter since Vanilla, I didn’t even know that… At the same time, I only feed my pet with the best food available at my level, even if he is already happy. Good booooy
There is a maximum level of happiness, after that you are wasting food.
Lets say you pet is happy at 75%, you give it food again and it will stop at 100%
Nothing is wasted when you have to please your Good boi
Until their legs don’t touch the ground or they can’t fly anymore.