Hi Bernhard!
Surveys were updated to be automatically responsive on January 25th, 2015. When you create a Survey in CRM, there will be a field on the Survey Record called Web Responsive. This determines whether you would like your Survey to be responsive and scale automatically based on the device it is being viewed on. By default, this field is set to Yes.
If your Survey was created before this update, it will not be responsive automatically. However, once you open the designer of the Survey and press Save, it should be updated to become responsive.
If you are still having trouble with your Survey being responsive, I would recommend that you open a support ticket so that we can get the URL for your Survey and further investigate.