>> a simple 1024x1024 atlas adds 12+ megabytes to live bytes
Thats about right for uncompressed image data.
Maybe you are getting confused because when the image is compressed to png format it is much smaller on disk, maybe 1.5 mb depending on complexity of the image. But to use the image it has to be decompressed in RAM to its true size which is about 12 megabytes it seems.
Incidentally you asked:
>>Is that because of sparrow?
No, it isn't!