As mentioned in the question, I build a kivy app and deploy it to my android phone. The app works perfectly on my laptop but after deploying it the font size changes all of a sudden and become very small.
I can't debug this since everything works fine. The only problem is this design or rather the UI.
Does anyone had this issue before? Do you have a suggestion how to deal with it?
PS: I can't provide a reproducible code here since everything works fine. I assume it is a limitation of the framework but I'm not sure.
It sounds like you coded everything in terms of pixel sizes (the default units for most things). The difference on the phone is probably just that the pixels are smaller.
Use the kivy.metrics.dp helper function to apply a rough scaling according to pixel density. You'll probably find that if you currently have e.g. width: 50, on the desktop then width: dp(50) will look the same while on the phone it will be twice as big as before.

PS: I can't provide a reproducible code here since everything works fine.

Providing a minimal runnable example would, in fact, have let the reader verify whether you were attempting to compensate for pixel density.
Why does the dimensions of Kivy app changes after deployment?