I'd guess some votes in Colchester's favour are based on the photo in this thread and not having seen the exhibit in person. It's not bad by any stretch, and as
@pipaluk states, it has certain viewing advantages over other exhibits. However, I would argue the enclosure's positives are somewhat undermined by its location next to the road train station, which is often busy and loud, negating the privacy offered by the offshow housing (which many other collections likely offer anyway). In addition, viewing surrounds the main enclosure space from all sides, again reducing private/retreat space.
This is absolutely true; there are far more factors influencing welfare than simply enclosure size. However, for a species that has a wild range of 4-10 square kilometres, is highly active and lives in large groups, I would argue that size is a major factor in this instance.