Recycling Bitmap does not free memory

I have an Activity in a TabHost with 3 other activities. Hence, these are always alive (or in "on pause" state).

The first activity has four different images (~250kb each) and they are retrieving a lot of memory (around 80MB. Just to point out, I load the minimum size needed for the screen and I'm using layout_weight if that helps), so I want to minimize the amount of memory which is needed.

I already tried to delete the images on the OnPause state and set them again on OnResume, but I didn't have luck, this is one example of what I was trying to do:

 imageView.Drawable.Callback = null;
 ((BitmapDrawable)imageView.Drawable).Bitmap.Recycle();
 imageView.Drawable.Dispose();
 imageView.SetImageDrawable(null);
 imageView.SetImageBitmap(null);
 GC.Collect();

I don't know if deleting the Bitmap on OnPause is the best strategy, but it should work. I don't understand why the ImageView isn't collected by the GC (since there are not external references)

EDIT This is how I'm loading the images. It doesn't work even if I put the images on the xml file. Besides, I don't care this code, I just want to dispose the bitmaps.

       void SetBackgroundImages(int imageId, int resId, float width, float height) {
        var imageView = FindViewById<ImageView>(imageId);
        using (var bitmap = DecodeSampledBitmapFromResource(Resources, resId, width, height))
            imageView.SetImageBitmap(bitmap);

    }
    public static Bitmap DecodeSampledBitmapFromResource(Resources res, int resId, float reqWidth, float reqHeight) {

    var options = new BitmapFactory.Options {InJustDecodeBounds = true};
    using (var b = BitmapFactory.DecodeResource(res,resId,options)){}
        options.InSampleSize = CalculateInSampleSize(options, reqWidth, reqHeight);
        options.InJustDecodeBounds = false;
        return BitmapFactory.DecodeResource(res, resId, options);
    }