Actually, a common misconception is that shrimps are smaller than prawns or worse (misconception) that "shrimp" = "small prawn".
In taxanomy (ie. biological classification), they belong to different sub-Orders, Families, Genus and Species. Some shrimp are bigger than prawn, and some prawn are bigger than shrimp.
But (and not just in Singapore, but in USA and everywhere else in the world), the common man-in-the-restaurant and restaurant menus themselves, don't give a damn whether it's "shrimp" or "prawn", and so mis-usage of these terms interchangeably is commonplace and can be forgiven.
Wikipedia :
While in biological terms shrimps and prawns belong to different suborders of Decapoda, they are very similar in appearance. In commercial farming and fisheries, the terms shrimp and prawn are often used interchangeably. However, recent aquaculture literature increasingly uses the term "prawn" only for the freshwater forms of palaemonids and "shrimp" for the marine penaeids [4].
In the United Kingdom, the word “prawn” is more common on menus than “shrimp”; while the opposite is the case in North America. The term “prawn” is also loosely used to describe any large shrimp, especially those that come 15 (or fewer) to the pound (such as “king prawns”, yet sometimes known as “jumbo shrimp”). Australia and some other Commonwealth nations follow this British usage to an even greater extent, using the word “prawn” almost exclusively. When Australian comedian Paul Hogan used the phrase, “I'll slip an extra shrimp on the barbie for you” in an American television advertisement [5], it was intended to make what he was saying easier for his American audience to understand, and was thus a deliberate distortion of what an Australian would typically say.
http://en.wikipedia.org/wiki/Shrimp
http://en.wikipedia.org/wiki/Prawn