I read from a website that Debian with LXDE is more lightweight than Lubuntu although lubuntu's size is 1 GB and debian's is 1.9 GB? How could it be? Is it realy lighter than Lubuntu? Which one contain more packages? Does the lightweightness depends on size?
Lubuntu tweaks LXDE to make it work and look in a somewhat different way compared to 'vanilla' LXDE. Debian-LXDE is much closer to the 'vanilla' LXDE.
Light-weight can mean different things.
RAM usage: You can define the footprint of a desktop environment by how much RAM it needs to run idle, and how much its default tools need. In this case you can compare
lxdeof Debian. You can use various tools for this task, for example
free -m top htop
Size of the iso file or installed system on disk: The size of the installed system is the size of a fresh installed system (before you have added files manually) as reported by for example
If the iso file is bigger, there are more packages or bigger packages or both.
I consider RAM usage the main criterion to decide the footprint or lightweightness of a desktop environment.