I'm on a panel very soon (within an hour) called "Is anthologising a critical act?" Hmm, that's a slightly oddly-worded topic, but I've been thinking a lot lately about the process of putting together an anthology. It doesn't seem right to me to talk about it as a "critical" act, exactly. It's a creative act of a kind, but in some ways more like creating a small business from scratch than like writing your own book. The overall effort in putting together an original anthology such as 50 Voices of Disbelief is, however, certainly comparable to that of writing a book of similar length.
The small business side of it is shown by (for example) the sheer number of emails it generates - to and from potential authors, the actual authors, various people at the publishing house, etc., and between the editors. My email folder for the book contains well over 2000 emails, going on 3000. I also suspect, by comparing other editorial work I've done, that the amount of work does not increase in a neat linear way with the size of an anthology: the bigger the book the more complex the task becomes, so a book twice as big requires a lot more than twice as much work.