Sounds like you've accidentally compressed it. Safari remembers what you've asked for from previous sessions.

To fix:

1) visit DU with Safari
2) Hit the "aA" button on the top left to bring up the site-specific settings
c) The top row will have a little A, a percentage, and a Bigger A.

Chances are your zoom percent isn't 100%. Hit the bigger A to make it bigger until you're back to 100%

Or .. it could be something else. Hope this helps.