Coinbase limits a lot of transactions based on how much personal information you provide. With just basic information provided, the amounts you can transact with are quite small. In order to move around larger amounts of money, Coinbase usually requires you to submit a picture of a government ID and your social security number. In fact, they might require a bit more information than a bank asks for:
http://insidebitcoins.com/news/coinbase-asking-for-more-information-than-banks/34244
Most likely larger transactions are already being reported to government authorities automatically for anti-money laundering purposes. It wouldn't surprise me if there isn't something that automatically flags you for an IRS audit or investigation for truly large money transactions.