Your premise regarding the market not being of sufficient size to support a 1M BTC transaction is flawed.
You're assuming:
1. That it would be a block transaction, where a block doesn't refer to a BTC block but a single transaction. Most institutional/sophisticated/smart traders scale out of their positions.
2. There may be an underlying assumption that just because someone (satoshi etc.) sell their bitcoin's for X price they would want to withdraw it in another currency. If the goal is to own more BTC, they would transfer in BTC to a trading account, sell said BTC while scaling out and buy back BTC when prices drop below their average sell price.
If someone was extremely cautious they would do these trades under multiple accounts, most BTC trading accounts need verification for deposits/withdrawal only. This could be done on multiple exchanges.
The average weekly trading volume of mtgox is >130,000BTC/week, therefore over 7 million BTC were traded which is sufficient volume to liquidate 70-100% of 1M BTC.