When does Safari on iPhone change the text size when the orientation changes? I've noticed that when I am browsing a web page in Safari on the iPhone and change the orientation from portrait to landscape, sometimes the text size and/or wrapping changes.

For example, in portrait there may be 10 words on a line, but when I turn it sideways the text size changes and there are maybe 8 or 12 words on that line. Usually when I turn it back to portrait again it stays at that number instead of going back to 10.

I don't remember any specific examples off-hand but they were just regular sites (not mobile versions). Text appeared to be in a fixed column, it wasn't just wrapping to the entire window width.

Can anyone clarify in what ways this is changed or determined?

[also, I couldn't create 'safari' or 'zoom' tags]
Here's an estimation:

The site you're likely viewing has what is called a liquid-layout, which means that the content will spread to fill it's container.  So when you're rotating the phone, the viewport width is changing from the narrower portrait width to the much longer landscape width.  So the text itself might not be changing size, the webpage is just adapting to fit it's new window size.

Luckily the above link also has a liquid-layout, so you can see this in action.  Take the window on your computer and shrink it's width.  You'll notice that the text will shift around to fit it's new container size.  So it's the web page (more specifically the CSS) which determins how the text wraps.

tl:dr

CSS did it.