Some font-size's rendered larger on Safari (iPhone)

Joe's response has some good best practices in it, but I think the problem you're describing centers around the fact that Mobile Safari automatically scales text if it thinks the text will render too small. You can get around this with the CSS property -webkit-text-size-adjust. Here's a sample of how to apply this to your body, just for the iPhone:

@media screen and (max-device-width: 480px){
  body{
    -webkit-text-size-adjust: none;
  }
}

Use 100% instead of None.

normalize.css includes this

Also, make sure you are setting the initial zoom setting to 1 in your viewport meta tag:

<meta name="viewport" content="width=device-width; initial-scale=1.0;" />