Buttons with dynamic images with kivy

Problem

I have a button with circular image, with the image in 1:1 ratio, and when executing the code (below) the program generates a screen with aspect ratio also of 1:1, but it is possible to resize the screen and change its aspect ratio and size, however, the image does not grow together as the screen and occupies a smaller proportional space. I tried to set values for size_hint:, but it distorts the image, less in a single aspect ratio. How can I make this button adaptable in relation to at any ratio and Size, without distortions and with the same proportional space?

Generated fabric (1:1)

Resized canvas (16:9)

Code

import kivy
kivy.require('1.9.0')

from kivy.app import App
from kivy.uix.floatlayout import FloatLayout
from kivy.uix.image import Image

class FloatingApp(App):

    def build(self):
        return FloatLayout()

flApp = FloatingApp()

flApp.run()

File .kv

<Botao@Button>:
    font_size: 32
    color: 1, 1, 1, 1
    size: 138, 138
    background_normal: 'bd.png'
    background_down: 'bd1.png'
    background_color: 0.88, 0.88, 0.88, 1  
    size_hint: None, None

<FloatLayout>:
    Botao:

        text:"Botao"
        pos_hint: {"center_x": .5, "center_y": .5}