I actually consider big nipples somewhat of a fetish of mine, and of the two sites posted, only a few pictures/girls even came close to actual big nipples. this is a good example of what I would hope to see on the other end of a big nipple link:
Link Hotten from:
http://www.voyeurweb.com
In my opinion, 'big' refers to the diameter of the aereola, and should be noticebaly darker than the girl's skin.
Carry on...