It's not low quality.
It just looks sharper on your site - are you running XFMG 1.1.6?
It's possible it's running through the new unsharp mask filter we added in XFMG 1.1.6. It's not supposed to be quite that noticeable on watermark resizes so we can look at adjusting it.
However, presumably you aren't going to be using the Google logo for real, so you may find the results are more desirable for your actual watermark. Watermarks that have a white outline with a darker inner colour generally produce the best results (and the unsharp mask filter shouldn't really have as much of an effect on that).