I believe that hydrogen is the smaller element: one proton, one neutron, one electron. Helium is an element, not a molecule: as I recall, it has two of each. Water is relatively (very) large in liquid or vapor form (it's a lattice when frozen). Aside from those minor (and admittedly annoying and pedantic) points, I speculate that the gas mixture in immersion chambers has, as with some inhaled decompression gas combinations, a substantially higher helium concentration (especially when compared to atmospheric "air"); hence the rationale for an escape valve in the case. Perhaps not. An interesting question, regardless!