Bitcoin mixing and transaction sizes
The mixing service could use various means to obfuscate the mixing - for example:


It can make you send coins to some off-line address, then send you your coins from its own supply of bitcoins before introducing your coins into the circulation again. This way it would appear that the transaction took place before you send the coins to the mixing service, and thus be harder to link
The service could split your Bitcoins into many smaller amounts and launder them each individually
It could deposit some of your money into some eWallets or similar services that use a shares pool of coins
It could use your money to purchase mining hardware and mint you fresh coins
The service could pay your money back to a few of your receiving addresses, making finding the final amount harder


Generally, there are many creative options of hiding things in the Bitcoin network, you just have to use your imagination.