right if you generate any new keys
i only have 2 wallet addresses in my wallet, no what i think has happened is that 12.1 added indexing to the wallet.dat file, meaning it doesnt take an hour now to load and reindex my wallet. Now all i need to do is make a larger truecrypt container
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Wallets also store information about transactions you received (and created). I have a wallet file with 100 private keys thats over 400 mb. That will not happen under normal circumstances (it was generated during a past spam attack on the network), but you should plan ahead for your wallet file to grow in size.