FS9 only allows bitmaps up to 1024px*1024px in size, FSX can have 512*2048 as an example (doubling on each other, or quadrupling), FS9 for high quality but frame hungry textures use 32-bit textures, i prefer DXT3 as a trade off between clarity and detail while saving frames, FSX same again applies except i prefer using DXT5 textures there,
Hope this helps,
AGNT_Matt