Source code for https://victorfont.com/resize-google-recaptcha-for-mobile/

resize_google_recaptcha.css

.g-recaptcha {
    transform-origin: left top;
  -webkit-transform-origin: left top;
}

resize_google_recaptcha.js

<script>
jQuery(document).ready( function ($) {
    "use strict";
    
    function scaleCaptcha( elementWidth ) {
        /* Width of the reCAPTCHA element, in pixels */
        var reCaptchaWidth = 304;
        
        /* Get the containing element's width */
        var containerWidth = $('#frm_field_xxx_container').width();
        
        /* Only scale the reCAPTCHA if it won't fit inside the container */
        if(reCaptchaWidth > containerWidth) {
            /* Calculate the scale */
            var captchaScale = containerWidth / reCaptchaWidth;
            
            /* Apply the transformation */
            $('.g-recaptcha').css({ 'transform':'scale('+captchaScale+')' });
        }
    }
    
    scaleCaptcha();
    
    /* Update scaling on window resize */
    $(window).resize( scaleCaptcha() ); });
</script>