I don't remember anything canon mentioning that Aura needs a lot of flesh to produce. That being said, it's certainly a philosophical and perhaps scientific thing for everyone including Gren to ponder. Gren was born a flesh and blood faunas, with his own aura and soul. How much of that is gone now that he's mostly a machine? Was it not his soul that granted aura, not his body? Now we can always argue that [url=http://tvtropes.org/pmwiki/pmwiki.php/Main/CyberneticsEatYourSoul]Cybernetics Eats Your Soul[/url], and to be fair Gren did take up additional robotic parts for the purpose of inhumane acts of violence, but he would have done that with flesh and bone if he had the patience to heal properly. Personally, I figure Gren falls under this [url=http://tvtropes.org/pmwiki/pmwiki.php/Main/TheseusShipParadox]trope[/url] instead. He's still Gren the Boar Faunas, just with new parts. 

Now if you're wondering how he keeps surviving these explosions, aura helps, but it's mostly because his body is built for such things. When you're essentially a walking wall with a gun, you have to be ready for explosions in the face. At least when he was testing his jetpack Gren could anticipate the explosions and prepare accordingly.