Kivy: size of layout doesn't update on app start

I am quite new to kivy and python, first question here... So, I want to hide the layout if the checkbox is empty manipulating size and opacity properties. It works as expected if you use checkboxes, but when app run, there is empty space. I tried to use constructor and schedule_once functions to change size to [0,0], and looks like size property changes, but still there an empty space you can see in screenshot.

P.S. I have a hypothesis, that the issue somehow related to kv file includes because I tried to create a minimalistic example for this question, and that... works fine, using just main.kv and main.py.

So, here the code I prepared to replicate the issue:

https://github.com/NobleFox/kivy-toggleLayout

Screenshot

Big thanks in advance!