When you zoom UIImageView object inside UIScrollView, you change only UIImageView frame, but UIImage in image property remains unchanged.
if you only want to show this image scaled in some other UIImageView, you should not change UIImage size, just set your new UIImageView frame same as frame of zoomed UIImageView from UIScrollView and use same image:
UIImageView * ImageViewOnScroll;
//set image and zoom it
...
UIImageView * newImageView = [[UIImageView alloc] initWithImage:ImageViewOnScroll.image];
newImageView.frame = ImageViewOnScroll.frame;
if for some reason you want to create new UIImage with changed size you can do it for example with following simple method:
UIImage * resizeImage(UIImage * img, CGSize newSize){
UIGraphicsBeginImageContext(newSize);
//or other CGInterpolationQuality value
CGContextSetInterpolationQuality(UIGraphicsGetCurrentContext(), kCGInterpolationDefault);
[img drawInRect:CGRectMake(0, 0, newSize.width, newSize.height)];
UIImage * newImage = UIGraphicsGetImageFromCurrentImageContext();
UIGraphicsEndImageContext();
return newImage;
}