It could be possible that the format was first very reasonable, but the surrounding platform has changed completely during the development. Then the new layers of specification have been added in a form that seemed to be the best possible solution on that platform and on that time. Wasn't Photoshop at the beginning an app for the original m68k Macintosh? Surely different kind of field sizes made more sense in that world than ours - also the tradeoffs in the sake of performance could have had some say.