I don't know about big and small games, he seems to suit playing against certain types of opponents. For example, almost all his very best games have come against Arsenal. Good ones against Chelsea, too. He does well against big European teams who attack quickly, like Milan and Bayern, rather than keeping and sitting on the ball. Gives him space to work in, and lets him play without thinking too much, which always seems to be his downfall.
Basically, if a match gets cagey and sluggish, so does Nani. If a match is a bit back-and-forth and played at a high tempo, he shines. Which isn't ideal going into a game against Liverpool's constant Allen to Skrtel to Agger to Allen passing game. I'd still be very happy to see him start, though.