The file size depends on: The amount of color depth you set/saved the document, the file format, image resolution, the amount of details.



Maybe try to save the image in a file format that compresses the file size, if that doesn't work try to reduce the amount of color depth.



This was a problem with my wallpapers too back then. All of my wallpapers I've made then had a file size of 30-35 MB. But now the file size of my wallpapers are 6-12 MB due to discovering how to save images in a PNG-24 file format. Seriously once I had a wallpaper that was 128 MB in file size. ><



I don't know what program you use, so the best I can advice is to experiment with different file formats until you find the best one.