I know how to fix it. I'm trying to understand why it happens in the first place.
But usability should trump aesthetics. If you're designing a blog, you should design it to be readable. That means a decent font size, no distracting graphics and definitely nothing that moves/blinks/flashes. (I'm biased, I've always preferred reading sites that are "plain." Just text and images only if they actually add value. Some color is ok too)
I looked at the source code for that page, and it switches from "12px" font size to a smaller version of that about halfway through. Aside from the issue of specifying font sizes in pixels, why would you suddenly start adding <small> tags to already-small text?
The small tags are part of the original article that he scraped. Either way, it's odd, the font size get progressively smaller for each paragraph.
This or another post proposed the hypothesis low-contrast, small text etc may look better during the design stage. Remember, the designer is probably working with dummy "lorem ipsum" text. Low contrast will highlight the designers contributions (color scheme, placement of info) and [save him/her the pain of reading the dummy text :)].