……. not what is biggest or on top.

A great example of using contrast to highlight your product comes from Coke and the Birth of the contour bottle .

“Bottlers worried that the straight-sided Coca-Cola bottle was easily confused with imitators. A group representing the Company and bottlers asked glass manufacturers to offer ideas for a distinctive bottle. A design from the Root Glass Company of Terre Haute, Indiana won enthusiastic approval in 1915 and was introduced in 1916. The contour bottle became one of the few packages ever granted trademark status by the U.S. Patent Office. Today, it’s one of the most recognized icons in the world – even in the dark!” (Coca Cola Company)

http://www.coca-colacompany.com/timeline-the-evolution-of-the-coca-cola-bottle#contour

In recent times products that have succeeded thanks in part to strong contrast at shelf from their competitors are Pringles Chips in the iconic cylinder and Garneir haircare which launched in bright green to stand out from its predominately white competitors.
However when trying to attract the shopper through contrast, its important to know that shoppers have an internal category code when it comes to products within a given category; deviation from this has the potential to cause confusion. (Block, 1995)

References
Block, P. H. (1995, July). Seeking the ideal form: Product design and consumer response. Journal of Marketing.
Coca Cola Company. (n.d.). History of Bottling. Retrieved April 10, 2014, from http://www.coca-colacompany.com: http://www.coca-colacompany.com/our-company/history-of-bottling