https://blockchain.info/wallet/paper-tutorial

All you need to access a certain wallet address is the private key. Create address/key pair offline, then print out the address and private key on paper, which gives you all the info you need to spend the BTC. Send BTC to this address and the only way to spend it is to know the printed details.

Your wallet can contain many different addresses and keys (behind the scenes) as you send/receive coins, and that's how they grow in size. This is why it's wise to make continual backups (or more often than every 100 transactions your wallet was a part of) in order to make sure you back up all the new addresses your wallet creates in the course of spending/receiving.