@ Arunil- The width of the continental shelf has to do with plate tectonics and plate boundaries. If you look at a map of the continental shelves, you will notice that they are narrowest around plate boundaries, especially subduction zones. They are thickest in the Polar Regions where there are no tectonic forces causing the plates to diverge and converge on one another.
They are also thick in regions where there are islands within a few hundred miles of shore and the shelves of these landmasses connect with the shelf of the continents. These regions around New Zealand, Japan, Indonesia, Madagascar and the Seychelles, the Caribbean, and the United Kingdom and northern Europe have bigger shelf seas for this reason.
Since these regions are less active seismically, there is less mass wasting and coastal subsidence to erode the coastal shelf. The result is a shelf break further away from shore.