Part of the reason cats (particularly big cats) do it a bit tough in captivity is because the enclosures are, of necessity, a bit smaller in area than is ideal.
I'm no expert either, Richard, but when I look at my old domestic cat at home I'm aware that she lives contentedly in a self-imposed area of about 800 square metres (being my backyard and the backyard next door.)
I know that if she was confined to a cage about the size of a bedroom (lets say approx. 12 to 15 square metres) she would not be content. So I feel that if cats big and small were given larger enclosures in zoos they would be more content.
I realise, of course, that it's not as simple as that. The "furnishings" in the enclosure are important, too - substrate, rocks, logs, vegetation etc. Also, the relationship with the keeper(s) has been shown to be important to the mental welfare of the cats (and indeed all animals.) At one time it was believed that the best way to breed cat species which were regarded as "difficult" was to keep contact with them to a minimum; then it was realised that the most success came when the cats had a good relationship with their keepers.