Here's where I think the problem lie. Give it a try and let me know if it helps.
If your iPhone is iPhone 4 or newer, you'll run into some problems with the using just media queries to change text sizes. This is because iPhone 4 and above have a "higher resolution" screen. Meaning to say 1px on the desktop doesnt equate to 1px on the phone. The actual iPhone 4 dimensions would be 640 x 960 instead of the usual 320 x 480 in the iphone 3.
There's one way to fix this though, and that is to set the viewport initial scale to 1.
Here's the code you have to place in the head of your HTML file.
<meta name="viewport" content="width=device-width, initial-scale=1.0"/>
If it doesn't work with this alone, try it together with ralph's suggestion and see how it goes.
Hope this helps!