Mobile Safari on iPhone will automatically increase the size of small text.The work around is ‑webkit‑text‑size‑adjust: 100%

iphone-text-resizing-disable.css

/*
  On mobile devices, the text-size-adjust property allows Web authors to control if and how
  the text-inflating algorithm is applied to the textual content of the element it is applied to.
  As this property is non-standard, it must be used prefixed: -moz-text-size-adjust,
  -webkit-text-size-adjust, and -ms-text-size-adjust.
  Reference:
  http://blog.55minutes.com/2012/04/iphone-text-resizing
  http://davemacaulay.com/webkit-text-size-adjust-doesnt-work-on-android-chrome
*/

p {
/* Text is never inflated */
text-size-adjust: none;

/* Text may be inflated */
text-size-adjust: auto;

/* Text may be inflated in this exact proportion */
text-size-adjust: 80%;

/* Global values */
text-size-adjust: inherit;
text-size-adjust: initial;
text-size-adjust: unset;

/* iOS, practical use */
-webkit-text-size-adjust: 100%; /* [chrome] */
-ms-text-size-adjust: 100%;     /* [IE] */

/* Android chrome, use max-height: 5000em; on parent; */
}