They have about 3 different spines they can play. They can afford 2 first elevens whereas Arsenal don't have anywhere near the same strength in depth.
I really don't think Barca is much deeper than Arsenal at all. They just have better players in general and some of Arsenal's best player seem heavily injury-prone.
Lionel Messi is really good, btw. I'm just hoping he's having an off day on June 12. Really tired of playing such good teams in the first matches of the WC. Spain, Argentina, and Argentina in the last three
Upon reviewing you are completely right. Arsenal possibly have a bigger squad actually but Barcelona have at least 9 midfield/attacker players who I would be pretty scared to see on the teamsheet if I was against them. Arsenal have maybe 4 and 3 of them were injured (Fab, Arsh and Van). It's an almost impossible task.
As an aside, anyone else impressed by Dani Alves? Probably my top wing-back in the world. He's just a machine.