if you remove the `col` variable ,and add the buttonscatter directly like in https://gist.github.com/allan-simon/6972475 then it's going to take the full screen

gistfile1.txt

import kivy
kivy.require('1.1.3')

import random

from kivy.lang import Builder

from kivy.app import App
from kivy.uix.boxlayout import BoxLayout
from kivy.uix.button import Button
from kivy.uix.floatlayout import FloatLayout
from kivy.uix.scatter import Scatter
from kivy.uix.widget import Widget



Builder.load_string('''

<Showcase>:
    canvas:
        Color:
            rgb: 1, 1, 1
        Rectangle:
            size: self.size

    BoxLayout:
        orientation: 'vertical'



<ButtonsScatter>:
    scale_min: 1
    canvas:
        Color:
            rgba: .5, .5, .5, .2
        Rectangle:
            size: self.size
    BoxLayout:
        size: root.size
        padding: 40
        Button:
            text: 'Hello'
        Button:
            text: 'World'

''')



class Showcase(FloatLayout):
    pass


class ButtonsScatter(Scatter):
    pass


class ShowcaseApp(App):

    def on_pause(self):
        return True

    def build(self):
        sc = Showcase()

        col = Widget()
        s = ButtonsScatter(size=(300, 200))
        col.add_widget(s)

        sc.add_widget(
            col
        )
        return sc


if __name__ == '__main__':
    ShowcaseApp().run()