How to prevent iTunes from downloading entire library?

Hello, I have a very large iTunes library (~90,000 songs) on my iMac and have iTunes match. This allows me to play to my music on my iPhone and macbook air. However, when I start to play an album or song on my macbook air (small hard drive), iTunes begins to download my entire library. Under Preferences, Downloads, the automatic downloads says it's for purchases, not my entire library - but is this option the reason it downloads my entire library?

Thanks