Buttons are different sizes in different browsers

Because the outer container is already set to 100%, all you have to do is set:

a.maxbutton-2 {
  display:block;
  width: any /* will default to 100% */
}

To make it the full-width. After that you can set any width you want. If the above gives the desired result you can place it inside the media query. But change your media query as it basically says, when the screen is a minimum width of 300px, use the CSS. All devices (pretty much) are wider than 300px, so the media query will always apply.