Do you mean that the file size is larger? Might be because of how JPEG compression works, the B&W adds "grain" to the image, and the images are sharp transitions from black to white, so harder to compress with JPEG and the file size is bigger. You can demonstrate this with color images too, take a shot of a blank piece of paper under good light so that there is little noise, then take a shot of something complex like dirt or a lawn or something with a lot of texture. The more complex shot will be larger as a JPEG since there is more information.