You can share your Folder(s) from your iMac and access them from Air.
As you say perhaps you should keep the heavy files on iMac and just the ones you need on MBA.
you can switch on sharing on iMac in System Preferences > Sharing > File Sharing
in the right pane set the folder you want top share and permissions.
Once you are done you should see the iMac's shares on MBA in Finder, left pane Shared.
As long as you are on the same network.
Only if you partition your drive to provide an independent portion of the drive for your MBA and only if both computers are connected to your local network. If you plan to transfer data back and forth then you will need to activate File Sharing on both computers. You will also have to designate the MBA's drive and the iMac's partition as "shared." Select the computer's disk icon on the Desktop. Press COMMAND-I to open the Get Info window. Check the box in the upper panel labeled, "Shared folder." See also, Mac 101- File Sharing.