Possibly images bought by hobbyists or bloggers who are home from their weekday jobs doing things for fun. I'm guessing that these are one-time or infrequent buyers and thankfully they know how to purchase images properly instead of just snagging them from the web for their blogs or community brochures, etc. Maybe freelancers who are working on the weekends.
There is also a theory that Google ranks web pages that have unique images higher than those that use images that are similar to others that are already online. They try to identify pages that create original content rather than those that are just copying or reposting that content.
I notice a lot of blog size downloads are first time downloads (also possibly coincidental).
