1、查看 AFNetworking 代码，里面的估算方法是
CGSize imageSize = CGSizeMake(image.size.width * image.scale, image.size.height * image.scale);
CGFloat bytesPerPixel = 4.0;
CGFloat bytesPerRow = imageSize.width * bytesPerPixel; // 2560
CGFloat totalBytes = (UInt64)bytesPerPixel * (UInt64)bytesPerRow;
NSLog(@"s1:%lu",(unsigned long)totalBytes); // 输出 10240
2、查看 CGImage 类，发现下面的方法：CGImageGetHeight和CGImageGetBytesPerRow。这样计算应该更准确
CGFloat cgImageBytesPerRow = CGImageGetBytesPerRow(image.CGImage); // 2560
CGFloat cgImageHeight = CGImageGetHeight(image.CGImage); // 1137
NSUInteger size = cgImageHeight * cgImageBytesPerRow;
NSLog(@"size:%lu",(unsigned long)size); // 输出 2910720
使用 Instruments 查看内存使用情况：
instrument.png
和输出保持一致。
3、搜索 stack overflow ，参考这里：
NSUInteger s1 = UIImagePNGRepresentation(thumbImage).length;
NSUInteger s2 = UIImageJPEGRepresentation(thumbImage, 1).length;
NSUInteger s3 = CGImageGetHeight(thumbImage.CGImage) * CGImageGetBytesPerRow(thumbImage.CGImage);
NSLog(@"s1:%u",s1); // s1 is the size of a .png image when saved to a file
NSLog(@"s2:%u",s2); // s2 is the size of a .jpg image when save to a file with best quality
NSLog(@"s3:%u",s3); // correct
