C＃在运行时使用大量RAM的位图图像

I made a tilemap generator today, and I noticed that when importing images of size 128x128 pixels and stitching them together (tilemap) into one large bitmap (8192x8192 pixels; 64x64 tiles) it uses ~250MB RAM when the output of the image to disk (BinaryWriter) is only <400KB. I don't understand why internally it's using so much RAM.I made a tilemap generator today, and I noticed