This is my work around:
1) Add a wrapper div to the recaptcha div.
<div id="recaptcha-wrapper"><div class="g-recaptcha" data-sitekey="..."></div></div>
2) Add javascript/jquery code.
$(function(){
    // global variables
    captchaResized = false;
    captchaWidth = 304;
    captchaHeight = 78;
    captchaWrapper = $('#recaptcha-wrapper');
    captchaElements = $('#rc-imageselect, .g-recaptcha');
    resizeCaptcha();
    $(window).on('resize', function() {
        resizeCaptcha();
    });
});
function resizeCaptcha() {
    if (captchaWrapper.width() >= captchaWidth) {
        if (captchaResized) {
            captchaElements.css('transform', '').css('-webkit-transform', '').css('-ms-transform', '').css('-o-transform', '').css('transform-origin', '').css('-webkit-transform-origin', '').css('-ms-transform-origin', '').css('-o-transform-origin', '');
            captchaWrapper.height(captchaHeight);
            captchaResized = false;
        }
    } else {
        var scale = (1 - (captchaWidth - captchaWrapper.width()) * (0.05/15));
        captchaElements.css('transform', 'scale('+scale+')').css('-webkit-transform', 'scale('+scale+')').css('-ms-transform', 'scale('+scale+')').css('-o-transform', 'scale('+scale+')').css('transform-origin', '0 0').css('-webkit-transform-origin', '0 0').css('-ms-transform-origin', '0 0').css('-o-transform-origin', '0 0');
        captchaWrapper.height(captchaHeight * scale);
        if (captchaResized == false) captchaResized = true;
    }
}
3) Optional: add some styling if needed.
#recaptcha-wrapper {
text-align:center;
margin-bottom:15px;
}
.g-recaptcha {
display:inline-block;
}