[Log in] is the main function of the 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 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.