>>967
CGSize scrollViewFrameSize = _scrollView.frame.size;
CGSize imageSize = _imageVIew.image.size;
CGRect imageViewFrame = CGRectMake( 0, 0, scrollViewFrameSize.width, (imageSize.height * scrollViewFrameSize.width / imageSize.width) );
_imageView.frame = imageViewFrame;

CGSize contentSize = CGSizeMake( _scrollView.frame.size.width, ... );
_scrollView.contentSize = imageViewFrame.size;

かな
UIScrollViewのコンテンツの横幅をフレームの横幅以下にしたらよろしい