dmcmullen – I tested a site with iPhone and iPad as well with the meta tag removed from header.php and did not encounter the 90% width issue you are mentioning.
But you did encounter it, you can change the value of initial-scale to 0.8 or so value to fix the issue. You can constrain how much the user can zoom in or out by setting the maximum-scale and minimum-scale properties. You can change the value of maximum-scale to say 2 and immediately notice that the user can now zoom in much more than what is possible if you set the value equal to 1 as set by the theme.
Likewise, you can change the value of initial-scale to something like 0.5 and make the webpage stay centered in the middle occupying only half of the available screen width.
Avoid using device-width value and leave the browser to decide on the width automatically based on the content width. Hope this helps.