How can I make this SVG mask responsive and keep the shadow at the bottom right of the circle?

I have the below SVG which I am using as a mask to overlay on an image, I am trying to make the circle responsive, so it shrinks on mobile devices but haven't had any luck using view box etc as it removes the mask, I am also trying to position the shadow to the bottom right of the circle, but again I am having trouble making it responsive.

Any help would be greatly appreciated.

svg {
position: absolute;
top: 0;
left: 0;
height: 100%;
width: 100%;
}

<svg version="1.1" xmlns="http://www.w3.org/2000/svg" xmlns:xlink="http://www.w3.org/1999/xlink" viewBox="0 0 2000 1000" preserveAspectRatio="xMidYMin slice">
<defs>
<mask id="mask" x="0" y="0">
<rect x="0" y="0" height="100%" width="100%" fill="#fff"/>
<circle cx="50%" cy="50%" r="250" />
</mask>
<pattern id="img" patternUnits="objectBoundingBox" x="0" y="0" width="100%" height="100%">
<image xlink:href="https://images.unsplash.com/photo-1507181179506-598491b53db4?ixlib=rb-0.3.5&ixid=eyJhcHBfaWQiOjEyMDd9&s=299b6fae13eb39086f5bb28029c61760&auto=format&fit=crop&w=1778&q=80" width="100%" height="100%" preserveAspectRatio="xMidYMid slice" />             
</pattern>
</defs>
<path id="Shadow" d="M462.231,0c19.593,38.355,29.045,88.448,36.926,138.3,9.125,75.59-104.323,237.333-194.755,250.24C246.323,389.921,51.569,371.964,0,313.148c46.559,91.147,141.283,153.59,250.655,153.59,155.395,0,281.365-125.97,281.365-281.365A280.22,280.22,0,0,0,462.231,0" opacity="0.2" />  
<rect width="100%" height="100%" mask="url(#mask)" fill-opacity="1" fill="url(#img)" />
</svg>

Solution

Putting the path inside a g container allowed me to scale the shadow relative to the mask, hope this helps someone in the future.

<svg version="1.1" xmlns="http://www.w3.org/2000/svg" xmlns:xlink="http://www.w3.org/1999/xlink" viewBox="0 0 500 500" preserveAspectRatio="xMidYMid slice">
<defs>
<mask id="mask" x="0" y="0">
<rect x="0" y="0" height="100%" width="100%" fill="#fff"/>
<circle cx="50%" cy="50%" r="16.5%" />
</mask>
<pattern id="img" patternUnits="objectBoundingBox" x="0" y="0" width="100%" height="100%">
<image xlink:href="https://images.unsplash.com/photo-1507181179506-598491b53db4?ixlib=rb-0.3.5&ixid=eyJhcHBfaWQiOjEyMDd9&s=299b6fae13eb39086f5bb28029c61760&auto=format&fit=crop&w=1778&q=80" width="100%" height="100%" preserveAspectRatio="xMidYMid slice" />             
</pattern>
</defs>
        <g>
          <path id="Shadow" d="m 462.231 0 c 19.593 38.355 29.045 88.448 36.926 138.3 c 9.125 75.59 -104.323 237.333 -194.755 250.24 c -58.079 1.381 -252.833 -16.576 -304.402 -75.392 c 46.559 91.147 141.283 153.59 250.655 153.59 c 155.395 0 281.365 -125.97 281.365 -281.365 a 280.22 280.22 0 0 0 -69.789 -185.373" opacity="0.2" style="transform: scale(0.3) translate(579px, 650px);" />
          <rect width="100%" height="100%" mask="url(#mask)" fill-opacity="1" fill="url(#img)" />
        </g>
</svg>