Car starts after 2nd try?

I have a 2002 ford explorer 4.0 and i recently changed the fuel pump and filter. And i was why the car takes about 2 try's to start? The engine turns but doesn't turn on but after 2 or 3 try's it starts? Could it be the the battery in the car is a little smaller then the one that was in it originally?

Make sure there's a non-return valve fitted.

The battery would not cause that. It is a Ford, be happy that it starts at all.