Original image plus two (or more) scaled thumbnails. Also, I don't think they're doing any reuse (upload the same pin for 2 different S3 objects of the same image). 5.3Mb / object is still really high though.
Wow, if they are not doing any reuse that really really dumb. I can't back this up, but would have thought at least 50% of all pins are repins.... if that 410TB figure is correct, this oversight is costing them $18k a month. It would be simple to have a separate asset id and pin id...