Both Malte and Action9000 brought up what I believe the best reason as to why zombies are so big. To me, the idea of a zombie apocalypse let's people imagine themselves in a almost identical world without any complicated structure or mundane work routine, like an escape from your current problems to think "what if?". I don't take it to heart but I've joked around with my co-workers numerous times when bored at work with the scenario of a zombie apocalypse starting at the very moment. What would we initially do? What are short term and long term plans? It can be fun to come up with how things can play out. It does also allow for the idea of taking out humans without the associated guilt while banding up with your group of friends.
I've actually had the random chance of talking to a nurse about zombies and how he has had a lot of people ask if zombies could actually happen. His response was definitively no as once a body first becomes "dead" rigamortis sets in fast and muscle tissue fuses to the bone so a zombie wouldn't be able to even get back up.
