Text over dark backgrounds

I'm using Movie Studio HD Platinum 11, and when I render text over dark-colored backgrounds I get strange results. First, I create some text in Photoshop, then I add it into a Vegas project over a solid color background (like black). When I create HD files (like 1280x720 or 1440x1080) the text looks as it should. But when I create 720x480 videos, the text suddenly looks blocky and jagged. This also happens when I use logos with transparent backgrounds.

I have made sure that my text file is sized to match the video settings, and I make sure that my project settings match the settings of the file I am trying to make. Yet, every time I try to do this process in HD it works, and anything in standard definition looks jagged.

What am I doing wrong?