Setting the button's ImageView's ContentMode
will adjust the image's representation.
I used the code below to display two buttons. One has set the ContentMode and another doesn't:
UIButton btn = new UIButton(UIButtonType.Custom);
btn.Frame = new CoreGraphics.CGRect(50, 100, 100, 40);
btn.ImageView.ContentMode = UIViewContentMode.ScaleAspectFill;
ImageService.Instance.LoadUrl("https://img-prod-cms-rt-microsoft-com.akamaized.net/cms/api/am/imageFileData/RE1Mu3b?ver=5c31").Into(btn);
UIButton btn2 = new UIButton(UIButtonType.Custom);
btn2.Frame = new CoreGraphics.CGRect(50, 200, 100, 40);
ImageService.Instance.LoadUrl("https://img-prod-cms-rt-microsoft-com.akamaized.net/cms/api/am/imageFileData/RE1Mu3b?ver=5c31").Into(btn2);
View.AddSubview(btn);
View.AddSubview(btn2);
They have different effects:

The first button doesn't have enough width to display this image so it clips the left and right edges due to the configuration of content mode. But the second button stretches the image to fit the whole space. Both buttons have the same size.