If I understand this correctly, the blockchain transaction file is already pretty big (i.e. a couple of GBs). When you first setup Bitcoin on your machine you a

If I understand this correctly, the blockchain transaction file is already pretty big (i.e. a couple of GBs). When you first setup Bitcoin on your machine you are basically required to download this file, and it is then kept up to date using Bittorrent, presumably with partial updates? I believe that there is a way (within the bitcoin design), such that the file size can be limited, firstly by only downloading the transaction headers, and / or secondly, by removing historical transactions (by breaking off branches from the Merkle tree [http://bitcoin.stackexchange.com/questions/10333/blockchain-long-run-issue] http://bitcoin.stackexchange.com/questions/10333/blockchain-...). However, when do those measures come into play? Do they have to be switched on? If so how, the whole thing is decentralized? Are the full transaction histories already being pruned?