Default font-size of several form controls: 13.3333px in Chrome; 21.3333px in Firefox

Hello,


Chrome 67.0.3371.0's Inspector tool and Chrome 67.0.3371.0's user agent 
style sheet reveal that default font size of several form controls is 
13.3333px:

input, textarea, select, button {
(...)
     font: 400 13.3333px Arial;
}


- - - - - -


Firefox 52.7.3 and Firefox 61.0a1 (today's nightly build)

input {
(...)
	font: -moz-field;
(...)
}

coming from resource://gre-resources/forms.css

and computes to 21.3333px

textarea {
(...)
font: medium -moz-fixed;
(...)
}
coming from resource://gre-resources/forms.css

and computes to 12px


select {
(...)
	font: -moz-list;
(...)
}

coming from resource://gre-resources/forms.css

and computes to 21.3333px


button,
input[type="reset"],
input[type="button"],
input[type="submit"] {
(...)
   font: -moz-button;
(...)
}
coming from resource://gre-resources/forms.css

and computes to 21.3333px

- - - - - -

Why is there so much difference with regards to default font size of 
several form controls between Firefox and Chrome?

Why not use and rely on user's preferred font size instead? ... which is 
what I would expect. If my preferred font size as set in browser 
preferences is 16px (or 17px or 18px), then I do not get 16px (or 17px 
or 18px) in both browsers.

Gérard