An attacker with 10,000 BTC could hire enough hashing power to perform a double spend for lets say 1000BTC (probably a lot less than this in reality if miners didn't mind leasing you hashing power), the other 9,000 BTC can be double spent such that the attacker ends up with 9000BTC of cash/goods/services and 9000BTC of bitcoin. The attacker has increased in worth by 8000BTC, the miners on average have increased in worth by (1000BTC - work done producing orphaned blocks) and a lot of merchants/exchanges/service providers are out of pocket by 9000BTC.