Well I have 6 wads: wad1: 200mb wad2: 24mb wad3: 62.1mb
wad4: 18.6mb wad5: 15.2mb wad6: 12.1mb
Maybe a ~300mb wad is too much for Hammer to handle?
Why not add them separately into Hammer?
edit: maybe I misunderstood you, anyway, I don't think Hammer likes big wad-files. A 200mb wad file doesn't sound logical to me, will you be using all those textures in your map? Why not just make a new wad-file that only contains the textures your map will be using?