if satoshi nakomoto mined 1 million bitcoins in early days. who own the second worlds biggest bitcoin wallet? 
I would guess one of the biggest wallets must belong to some exchange. I find it psycologically difficult, although not impossible, to hold the majority of coins past some price increase. almost everybody have some point on the price history where they would sell at least part of the holdings. sO at current prices the major holders are exchanges IMO.