They are both huge and considering the fact that Barca did worse at home than they did at Bernabeu, with the fact that Mourinho has played better games at home with the teams he managed (these could be manipulated in whatever way people like, I prefer it this way
), it could put Real in a good position. All depends on the outcome of the match itself.
Why does he feel the pressure ? Probably because it looks like he's concerned that Mourinho will put the mind games to work right before the game and he wants to make sure and do everything he can to stop him getting into his players' heads. Mourinho is a media beast and Pep actually does look like he is afraid of him affecting players' mentality which is crucial right now.
At least that's the way some madridistas could see it. A bit biased view, but who knows, that could be the reason for it