The players want to prove something to the world and this is why we haven't lost to any big team under Ranieri so far.
It's true but i'd add another reason for our good results vs the big teams, just as important as the motivation.
We all know that we're not a team that can have much possession. We score most of our goals out of nowhere. Camo to Trez, BANG. Del Piero from free kick, BANG. Nedved cross for Amauri, BANG.
We're a team well suited to play the game preffered by the serie A minnows. Defend with 10 man, hunt the opponents down all over the pitch and then strike on a counter or score from a set-piece. The difference between us and the minnows is that we have the required quality to hurt the opponent and create chances.
Ranieri noticed that we can't open up and let a strong opponent lots of space (like Real or Milan are leaving for their opponents). We don't have the required talent in midfield to play that game. He realized what's the kind of game we need to play against the big teams.
Just look at the 2 matches with Real and you'll see.
Good tactics+Motivation is the main reason for our great results vs the big guns.