This etymology is not certain. The ὑποβαρός can be either a deformation of the *ὑπόβαρις found in the Byzantine Etymologica (Gudianum, Magnum), and in that case this is the same etymology as ὕβρις / ὑφαιρέω, or be understood as an antiphrastic etymology "which has little weight", instead of "very heavy" which would be more appropriate. Maybe the first hypothesis is better. But the second etymology provided (ὕειν βάρος, probably corrupt) definitely involves βαρύς, βάρος, and the Pseudo-Zonaras understands it that way.