Question about textures in the image folder

[rstaats10]

I wonder why some bitmap textures in the images folder have dimensions of 256 x 256 and some in SEV have 256 x 256 or 512 x 512 or 1024 x 1024. When I add a ship from SEV to Starfury I can reduce the dimension to 256 x 256 and everything seems ok. This obviously reduces the file size but does this have any impact that I can't see? Why would we want the bitmap texture to be larger than 256 x 256? Also by making a smaller texture file does this help the game run better by decreasing the bitmap useage. If anyone can help me answer this I would appreciate it. I will be gone this weekend but I will check the forum when I return. Thanks