Nature conservation has to take place where there is still wilderness. Zoos make a contribution by keeping animal species in order to save them from extinction.
In the zoo live animals are often called to be „in captivity“. But zoo animals feel secure in their enclosures and do not know absolute freedom in nature either.
Animals have certain requirements regarding to how their artificial zoo environment should be designed. However the size of the enclosure is not that significant.
The fact that zoological gardens try to protect endangered species from extinction through breeding programs is well known. But they also have other tasks.