"One size smaller" - The essence of Symbio Eartips is that the foam part have its own inner force to push the silicone flange to the ear canal walls. Someone can think that Symbios are bigger than usual eartips... They are not bigger. After a lot of experiments we decided about this density of the foam part. If the density would be lower the the isolation won't be as good as now and Symbios wouldn't be Symbios.