The "Terrain 100%" means that your animal is completly fine with your choice of terrain composition...
it is just not enough there of it.
That been said, the needed space things seems to be quite... fluid.
My crocodile needed 210m land and 210m water.
I added 1 of them and the amount doubled.
Thats not always the case.
Warthogs was fine with their shown number of space until they became 5 or 6 iirr.
My 3 hippos got 1000m land and 1000m water and was happy about it.
Then they had 1 offspring and suddenly demanded 2000m land and 2000m water.
Maybe there is a formular for each animal, but it is beyond my poor little wolfbrain.
I guess it is always a good advice to plan double the amount of space the game shows you as needed.