By default, IIS doesn't map the MIME type for SVGs. You will have to update your Web.config to include the correct mappings for SVGs like so:
<system.webServer>
<httpCompression directory="%SystemDrive%\inetpub\temp\IIS Temporary Compressed Files">
<scheme name="gzip" dll="%Windir%\system32\inetsrv\gzip.dll" />
<staticTypes>
<add mimeType="text/*" enabled="true" />
<add mimeType="message/*" enabled="true" />
<add mimeType="application/x-javascript" enabled="true" />
<add mimeType="application/atom+xml" enabled="true" />
<add mimeType="application/xaml+xml" enabled="true" />
<add mimeType="*/*" enabled="false" />
<!-- HERE -->
<add mimeType="image/svg+xml" enabled="true" />
<add mimeType="application/font-woff" enabled="true" />
<add mimeType="application/x-font-ttf" enabled="true" />
<add mimeType="application/octet-stream" enabled="true" />
<!-- HERE -->
</staticTypes>
</httpCompression>
<urlCompression doStaticCompression="true" doDynamicCompression="true"/>
</system.webServer>
For more details on this, check out this answer. To test whether compression is working or not, use the Chrome Developer Tools and check the HTTP response header contains the following:
Content-Encoding: gzip