For development environment 150 MB is fine. The folder size is big because by default sencha cmd copies the framework code in the project directory.
It is not recommended to put this all files into final build.
Sencha cmd provides another useful command
sencha app build
which will generate the minified production build in build folder which is smaller and considered as code to publish.
To generate development, build without minified code you can try following command:
sencha app build development
You can find more featuers provided by sencha cmd at : http://docs.sencha.com/cmd/guides/extjs/cmd_app.html