Part of what makes Lubuntu so lightweight is the fact that it uses Openbox as its window manager instead of Compiz which is used by Unity. To ensure you have the lightest weight possible (and therefore Lubuntu instead of Ubuntu + LXDE), you need to install Openbox. It may be installed automatically when you installed
lubuntu-desktop. If not, you can install it with:
sudo apt-get install openbox obconfig
If it's already installed then running that command will tell you it is already installed.
If you want it to be even more lightweight at the expense of beauty and features, you can run Openbox by itself (without LXDE):
Edit your ~/.xinitrc file and add this line to trigger Openbox when X starts.
This page will help you with installing/configuring/using Openbox.