That's literally the size the graph came in. Check the properties and see for yourself.
Maybe the problem was that you didn't know about html size controls:
<img src="http://www.census.gov/indicator/www/img/c20_curr.gif" border="0" width="350" height="236">
--the last numbers in quotes being how big it would be at first. The advantage is that then you can control size in your post, and good etiquette is to be considerate so any other person you care to impress can right click and get if full size only if there's interest. Otherwise, you're just offending people by flashing stuff that nobody's impressed with.
We're talking about forum standards aren't we?