The following code will result in a small label and a big button:

from tkinter import *
from tkinter.ttk import *

root = Tk()

root.option_add("*Font", "sans-serif 12")

s = Style()
s.configure('TButton', font=('courier', 40))
s.configure('TLabel', font=('courier', 40))

Label(root, text="lbl").pack()
Button(root, text="bttn").pack()

root.mainloop()


It seems to me that both should have the same style. Removing the root.option_add line fixes it of course but this is a stripped-down example and in other cases it might be needed.