I've tried using pyinstaller to package a simple hello world example using both pyqt and wxPython and both of them were about 15mb to 20mb. Why is it so big , its not practical for simple apps. Is there any other cross-platform solutions?
For small applications you should take a look at Tkinter (or tkinter in python3). As it is part of the standard library, your clients don't have to install an additional library to use the graphical interface. IDLE, the ide which comes with the standard installation of python is written in tkinter for example.
To get started with tkinker, https://wiki.python.org/moin/TkInter is a good point to start.