@comradestryker All wrecks start with more hp that the unit had while alive. Often, however, they take damage. T3 power generators in a grid will do this, which is why sever grid damage usually wont leave very many wrecks, as the death weapons of nearby generators and fabricators destroy them.
It's also worth nothing that overkill damages also is transferred to the wreck. A t1 unit killed by a TML wont leave a wreck as the overkill damage kills the unit and the wreck in the same shot.
Lastly, remember that damage wrecks also lose value. A wreck that loses half It's hp also loses half the mass value.
I think it makes sense, from a physics and lore perspective, that wrecks have much higher hp that units, as their entropy is much higher. They are more like a mountain - which is basically the same even if its parts are rearranged - than like a complex machine which can only function if all the parts are arranged in a specific, planned way.