My first Mao game

I just had my first Mao jungle game in ranked, and I built absolutely no resistance items right off the start and I was tankier than anybody on the enemy team. I actually 4v5'd later on just with thornmail and that one jungle item every tank gets. So why does get to be so tanky without any items even assisting him? (My previous game I played trundle with as many defensive items as possible and I even ulted the enemy Mao, and he was still tankier even if I had more items =l)