frameForImageInImageViewAspectFit metotu ile aspectFit ile değişen boyutu yakalayabilirsin.

UIImageView+Extension.swift

extension UIImageView{
    func frameForImageInImageViewAspectFit() -> CGRect
    {
        if  let img = self.image {
            let imageRatio = img.size.width / img.size.height;
            
            let viewRatio = self.frame.size.width / self.frame.size.height;
            
            if(imageRatio < viewRatio)
            {
                let scale = self.frame.size.height / img.size.height;
                
                let width = scale * img.size.width;
                
                let topLeftX = (self.frame.size.width - width) * 0.5;
                
                return CGRect(x: topLeftX, y: 0, width: width, height:self.frame.size.height)
            }
            else
            {
                let scale = self.frame.size.width / img.size.width;
                
                let height = scale * img.size.height;
                
                let topLeftY = (self.frame.size.height - height) * 0.5;
                
                return CGRect(x: 0, y: topLeftY, width: self.frame.size.width, height:height)
            }
        }
        
        return CGRect(x: 0, y: 0, width: 0, height: 0)
    }
}