Lightness of an AV has nothing to do with RAM usage... it is even better if an AV uses lot of RAM, it means signatures are loaded in the RAM instead of the disk (hence reducing the various I/O) so the AV and the system will respond faster.Very light? they use over 260 megabytes, thats as much as kaspersky and norton combined.
Some useless AVs tricks the average users by hiding their RAM usage, and people say "WOW it uses only 5mb of RAM!! awesome!!! best AV in the market 111!!!!!" ... poor people...