If you're running the "standard" Bitcoin-Qt/Bitcoind client, it runs as a "full node" and so keeps a copy of all transactions ever done on the Bitcoin network, so that it can validate that they're all correct and serve up the transaction history to others. If you don't need that, then you may want to try
a different client and see if it meets your needs better.
If the issue is just that you don't want it in your apps data folder, you can move it somewhere else and use the -datadir command-line parameter to have it use a different location. But it will still be that big.
