[Log in] is the main function of the [[ https://en.wikipedia.org/wiki/Special:UserLogin | login form ]], yet nowadays its [Join Wikipedia] button is taller in both Firefox and Chromium. #wpLoginAttempt is 20px tall, while #mw-createaccount-join is 24px tall. I think this happens the line-height within the vform is 17.25 while the line-height outside the form in the "call to action" is 22.4px.
The [Log in] button used to use class `mw-ui-big`, which overrode this height reduction. See [[ https://commons.wikimedia.org/wiki/File:Screen_Shot_2014-02-07_of_English_Wikipedia_login_form.png | this screen shot ]]. @jdlrobson removed it in d51772b6e ("Stop using mw-ui-big on login form buttons") . If we don't want it to have [Log in] a larger size, we certainly don't want it smaller.