found it on sofeminine:
I have overs and fluffed 3 times LOADS of overs girls have fluffed.
Fluffing occurs when the skin stretches and that stretched skin, forms new skin so it gets fuller (Fluffs) SO that means both overs and unders can fluff.
Obviously with unders when the muscle relaxes to accomodate the implant they will get bigger too.
The PS on Lookyourbest was asked this and his answer is here http://www.lookyourbestuk.com/forum/viewtopic.php?t=28027
I had my BA a year ago and have come across loads of girls with unders and overs that have fluffed. It might not happen but usually does with unders because of the muscle.
Hope that helps.