Can anyone explain the combat scan view?

I don't understand why the green and red lines grow and shrink during the battle. What do they represent? Incoming/Outgoing damage? The total amount of hit points plus shields and armor? ??

thanks

I found this explanation: https://www.games2gether.com/endless-space-2/forums/65-general/threads/27040-scan-view-in-space-combat?page=1#post-242267

But I'm still confused. From my observations the bars grow and shrink so if it is meant to represend the damage dealt/received why does it shrink during battle?