The problem is the zoom settings specific to the DuckDuckGo site. The zoom level may be set between 50 and 300%. I would venture it is currently closer to 50%. You can access the Site Settings menu by:
While viewing the DuckDuckGo results page either...
- click on the "Site Preferences" icon (a gear wheel] or
- on the menu bar select Safari > Settings for this website…
…then click on the number immediately to the right of Page Zoom and select a comfortable zoom level for the specific site. That will not change the zoom level for any other website.
You can change the default zoom level for all web sites by going to Safari > Preferences > Websites and clicking on the Page Zoom icon
If you have problems with a particular web site, such as DuckDuckGO, in Safari > Preferences > Privacy you can find the problematic site and Remove it from the list which will automatically set it to the default values specified in Safari > Preferences > Websites.