Don't know if it's the same definition, but their GigaOM post defines light as: “light” AWS users — those with fewer than 8 instances
http://gigaom.com/cloud/newvem-pulls-back-the-curtain-on-ama...
Obviously it wouldn't make sense for people who only use micro instances, but then again people who only use one micro instance don't use a third party cost management tool.