The XNB file produced by the content pipeline contains decompressed image data, so its size is going to be a simple function of the texture's dimensions and bit depth. If you want the file sizes to be smaller, then either use smaller textures (that is to say, fewer pixels; whether you have 1 texture or 100 is irrelevant if they collectively contain the same number of pixels) or use a texture format that stores less data per pixel. You should be able to change the "Texture Format" option (under the content processor) for each texture to adjust its bit depth.
I would question whether this is really necessary, however. As pointed out in comments, this really shouldn't be that big of a deal, especially if the asset in question is your game's playing field. Is this a phone game?