关于UIImageWriteToSavedPhotosAlbum（:: :)方法

Question

使用UIImageWriteToSavedPhotosAlbum(:::)方法后，我有一张图片，尺寸为2347kb。使用照片查看大小737kb。为什么？我真的不知道。谁能告诉我？

代码：

NSData * data = UIImageJPEGRepresentation(image,1);
CGFloat length = [data length]/1024.0; // 2347 kb
self.showLabel.text = [NSString stringWithFormat:@"%.0fKB",length];
UIImageWriteToSavedPhotosAlbum(self.scalePhoto, self,@selector(image:didFinishSavingWithError:contextInfo:), NULL);

//result
- (void)image:(UIImage *)image didFinishSavingWithError:(NSError *)error contextInfo:(void *)contextInfo
 {
    NSData * data = UIImageJPEGRepresentation(image,1);
    CGFloat length = [data length]/1024.0; // 2347 kb
    NSLog(@"image = %@, error = %@, contextInfo = %@", image, error, contextInfo);
    if (error) {
        [Tools acquirePhotoAuth:^(BOOL grant) {
            if (!grant) {
                //setting
            }
            else{
               //error
            }

        }];
    }
    else{
        //success
    }
 }

为什么在桌面显示大小为737kb？