In the relationship hover it shows two aspects to compare but I'm not exactly sure what one of them means.  See below.  Under Strength, it says "Comparable" but under that it says you are Much Stronger than them.  My assumption here is that the "Comparable" more relates to science advancement and territory development and the Much Stronger relates to Army Size.  Is that the case? The wording needs to be more clear here. 

In Civ you could go to the Winning Conditions screen and see who was farther along in science and how much they were earning, likewise culture and religion.  You can see relative army sizes too.  I'm wondering how to do this type of comparison in Humankind.

The items you see under Strength are balanced against each other. So in this case, "We are much more powerful than them" is counteracted by "We depend on access to their resources". That's why it says Comparable.

As far as I can tell, the comparative power rating is based purely on military units, most likely the sum of your units' strength. I have seen my empire rated as less powerful, and after spending 2 turns pumping out units, suddenly flip to more powerful. Ditto for a round of upgrades.