Create a weight attribute in all the <node> tags.

Here is the GEXF documentation: https://gexf.net/data.html => to make it working, we have to:

• declare a <attributes> tag under <graph> (same level as the <nodes> tag)
• declare a <attvalue> under each <node> tag and specify the right value (this value is the same as the existing value of the value attr in the <viz:size> tag)