您可以尝试使用nuget Xam.Plugin.Media来设置压缩质量，以拍摄照片和压缩以及。
请参考以下代码：

private async void cmdCameraPhotograph_Clicked(object sender, EventArgs e)
    {
        if (CrossMedia.Current.IsTakePhotoSupported)
        {
            var file = await CrossMedia.Current.TakePhotoAsync(new StoreCameraMediaOptions
            {
                Directory = "Photographs",
                SaveToAlbum = true,
                CompressionQuality = 40,
                CustomPhotoSize = 35,
                PhotoSize = PhotoSize.MaxWidthHeight,
                MaxWidthHeight = 2000,
                DefaultCamera = CameraDevice.Rear
            }).ConfigureAwait(true);

            if (file != null)
            {
                
            }
        }
        else
        {
            await DisplayAlert("Not Supported", "Your device does not support this feature.", "OK. Understood")
                .ConfigureAwait(true);
        }
    }

您也可以从库中获取文件

var file = await CrossMedia.Current.PickPhotoAsync(new PickMediaOptions
        {
            CompressionQuality = 40,
            CustomPhotoSize = 35,
            PhotoSize = PhotoSize.MaxWidthHeight,
            MaxWidthHeight = 2000
        }).ConfigureAwait(true);

您也可以在不同的平台上单独压缩图像。要了解更多信息，请查看线程Compress images。