it seems that they all have huge capacity. I shall never use 320Gb let alone 500gb or 1Tb. It's not only MacDonalds who do 'Supersize' portions!
That's the trend .... bringing down the cost per GB and as a result being able to provide larger capacities as costs that you could have only dreamed of a few years ago.
The floods in Thailand last year have more than doubled the price of most hard drives, assuming you can get them
Supplies are limited and some wholesalers are rationing them
Only certain brands and types are in short supply.... plenty of others around.
Western Digital for example don't seem to have much of a supply chain issue, their WD5000AAKX model (500GB) for example, is in plentiful supply and dirt cheap.