Umutcan It just has some hardcoded values, instead of using the ones inherited from oauth, simply adding this would fix the font size issue:
.LogInButtons:not(.FoFLogInButtons--icons) .LogInButton--apple, .LinkedAccounts .LogInButton--apple {
font-size: inherit!important;
}
Maybe I'll try to send a PR, but before that I think it would be more interesting to send one for oauth and fix the off-centre icons/text.