Not lighter, but the Black Diamond BBEE 11 should be smaller. I have an older BD pack that is similar, which I like for day hikes when I don't need too much stuff. The BBEE seems to have a padded back panel which mine doesn't, but I usually add a small piece of blue foam for some structure, which is also a sit pad. BD is a climbing company so they tend to use more durable fabrics. I don't really need that much durability, but I also don't care that much about a few extra ounces for a day hike when I'm not carrying much weight anyway.

There is also the Gossamer Gear Riksak 17, which seems to be heavier than the Sea to Summit, but maybe a little smaller.

The ZPacks Zero is 3.5 oz and 16 liters.

When 3.5 oz is too heavy, you need the help of the SUL forums on Backpacking Light. I recall someone who make a "pack" that was 1 oz by sewing it to his shirt so he didn't need a back panel. You have to get creative at those low weights.