Looking for "Second Life" I see a thread that goes back to 2007, so perhaps it's time to describe the current state of the art in Second Life breasts.
A major advance in realism, or perhaps verisimilitude is the better term, came with the "sculpted prim". Rather than being a simple geometrical shape, a sculpted prim (in the initial version at least) is like a sphere that you can poke in or out where certain latitude and longitude lines intersect. (Right now there's a move underway to allow mesh import, which I expect will make things even better.) There are a fair number of vendors of prim breasts.
(By the way, prim breasts aren't just for those who want to have enormous breasts. The high end of the size scale for the stock Second Life avatar looks really bad--does anyone, save maybe the Bizarro World version of Lois Lane, have breasts with corners?--so prim breasts allow a pleasing shape over a wide range of sizes, and, at least up until a recent version of the Emerald Second Life client, it's been the only way to jiggle.)
Prim breasts can be scripted, so that they can grow or shrink on command, and even lactate (using "particle generators"). These days, they often come with a heads-up display to control position, size, clothing (a layer around them that can be made opaque or transparent), lactation, and even skin texture and color. Implant Nation (a name I really don't care for--prim breasts are made necessary by the limits of the SL avatar, but I don't like to think of them as implants!) was the first, as far as I know, to create prim breasts with HUDs; nowadays they are a common feature of prim breasts.
Prim breast makers typically provide some tops, and Smashing Prims was a pioneer in making really nice clothing that came with prim breasts that had matching textures, but the use of a clothing layer has made it possible for third parties to create clothing for use with prim breasts, and there's now a flourishing group of makers of such clothing. (I'm hoping that it will expand further, and that there will be more modest clothing to choose from as time goes on.)
You've already heard of Big'Uns and EPIC. There are other establishments dedicated to well-endowed dancers, though to the best of my knowlege, Hooterville, mentioned in another thread, is no more.
There's also a lot of Second Life based machinima--perhaps it would be a good medium for those wanting to do video involving breast expansion?
P.S. Second Life, with its emphasis on user-generated content, is the only virtual world I know of that caters to those who wish to have figures that vary from the mainstream. (Admittedly I'm not totally familiar with the field--I may try Red Light Center, if it will run under WINE (it's Windows-only... ptui!) to see whether it allows one to be well-endowed.