I think enclosure size is an interesting point of discussion when it comes to Marwell.
Yes, making an enclosure bigger would be making it better. However, look at the threads debating the best or favourite zoos in the country. There are zoos smaller than Marwell with more extensive animal collections that are frequently rated higher. That does imply that some zoos with typically smaller enclosures are considered to be better.
I do feel that the size of some of the enclosures at Marwell do limit the scope for the animal collection, even for a 140 acre zoo. It should be obvious which exhibits are the greatest contributors to that statement. It is also a generalisation and it doesn't mean there aren't individual exhibits at Marwell that could be larger. If you compare Marwell's tiger exhibit to other zoos, more modern ones are typically bigger.