My guess is that a wild gorilla is very bloated from eating very large quantities of vegetation with low nutritional value. Zoo gorillas eat a lot of vegetables and fruit that have much more calories, which means zoo gorillas eat a lot less, and they eat things that are easier to digest and cause less gass = smaller belly.
yes this bloated-belly image is exactly what a mountain gorilla should look like, and it is because they feed almost entirely on vegetation. Lowland gorillas eat more fruit and as Pertinax said are a little slimmer -- but still much bigger bellied than zoo gorillas which are actually "abnormal" in appearance.
Mwirima - the gorilla above - died on the 3rd of March. He has been sick for the last couple of weeks and not eating. I am very glad I saw him last year.
