size of .PNG file was increased after save back from NSView

I read a png file and save back, but size of it was increased even though they have same picture
Could you help me to fix bug of below code?

FCImageView *imgView = (FCImageView*)(tabItem.view);
CGImageRef cgRef = [imgView.imageView image];
NSBitmapImageRep *newRep = [[NSBitmapImageRep alloc]initWithCGImage:cgRef];
[newRep setSize:[imgView.imageView imageSize]];
NSData *pngData = [newRep representationUsingType:NSPNGFileType properties:nil];    

//write file
NSString *filepath [email protected]"/Users/apple/02_workspace/testSaveFile.png";
[pngData writeToFile:filepath atomically:YES];

In above code, [imgView.imageView image] return CGImageRef instance