That picture was taken September 18th, so the end of the season.
The variety of petunia makes a huge difference. You want "free flowering" trailing types. This includes all the Waves, Ramblin' Avalanche and Supertunias.
Petunias are fertilizer junkies. Water lots and fertilize lots. Maximum bloom with high middle number in fertilizer.
That's really how you get the big BALLS. Note: The pink petunia in the urns beside the table is one plant per pot. Hot Pink Tidal Wave
