The properties of steam table is a useful chart as it shows the relationship of pressure to the amount of cubic feet of space the steam will occupy. It can be clearly seen that the lower the pressure the more space the steam occupies. You can also see the latent heat available at that pressure. While there may be more btu's of latent heat available, it is spread out over a greater area. For a larger copy of this chart click on the link and it will come up in adobe format. Properties of Steam Table from the 1935 Hoffman Data Book, reprinted with permission, Courtesy ITT.