Hi Marissa,
Sorry for the inconvenience caused. From your description, you find that when you select Fill color option for Button appearance, you found that the height of the buttons is getting higher compare to “Outline” button appearance. We have also test on this situation from our side. We found that the height of the buttons is changed as your description as well which the height of the buttons was increased. I understand the changes on the button size may not fit to your site design. I suggest you can send this feedback via SharePoint UserVoice, which it is the most efficient way to let the relevant product team aware this problem. Thank you for your understanding.
Best regards,
Dihao