Okay, so here is how I solved this!

YouTube Video for this!

1. Open the SVG file
2. The SVG file Width and Height - set these to auto!

I've been having the same issue all morning and was about to completely give up on Svg support, seems mad that you can't get a scalable format to scale properly...

But, I had one more go and I think I've worked this out.

It seems that you need to tell the SvgImageSource to rasterize at the SVG's original design size and then get the Image to scale it. Not sure it's a particularly helpful approach, but certainly solves it as of build 15063.

<Image Width="24" Stretch="Uniform" VerticalAlignment="Center">
  <Image.Source>
    <SvgImageSource UriSource="ms-appx:///Assets/salesorder.folder.plain.svg"
                    RasterizePixelHeight="48"
                    RasterizePixelWidth="48" />
  </Image.Source>
</Image>

So if the SVG was 48x48 we turn it into a bitmap at 48x48 using the RasterizePixelHeight and RasterizePixelWidth values and then the image scales that to 24x24.

Hope that helps.

Update I just re-read your question and realised that you were looking to increase the scale whereas I've been working to decrease it. Looks as though the technique still works, but as you scale up you're going to lose any sharpness of image due to the bitmap scale process. I think this points to a fundamental flaw in the current implementation. They seem to be rendering the svg as a bitmap, and we have to tell it the original size to get it to render properly, and then they allow the bitmap code to do the rest of the work.

I think it's somewhat debateable whether this is true support or an odd half way house. Somewhere someone suggested that Adobe Illustrator can generate XAML paths, I think I'm going to look at that to see whether I can get a better quality output, shame though because I really like Inkscape :-(

For me, it worked with modifying SVG file like this:

1. Add appropriate preserveAspectRatio property to svg tag. For me it was "xMinYMin meet".
2. Set viewbox property of svg tag to this template "0 0 ActualHeight ActualWidth", in my case it was "0 0 1050 805".
3. Set height and width of svg tag to "auto". Now svg element is relative to Height, Width and Stretch properties you provide in your XAML page or view.
4. It might be needed to rebuild the project for XAML Designer to take effect.

SVG File:

<svg preserveAspectRatio="xMinYMin meet" viewBox="0 0 1050 805" width="auto" height="auto" ... > ... </svg>

XAML File:

<Image
    Width="200"
    Source="ms-appx:///Assets/Photos/Illustrations/sample.svg"
    Stretch="UniformToFill" />