I have delineated the watersheds from DEM using r.watershed command. I have specified the Minimum Size of Exterior Watershed Basin as 1500. I have read the description of r.watershed as well. I am not able to understand why there is so much variation in size of watersheds. There are tiny watersheds even of the size of one cell. How can such small watersheds form after specifying criteria as big as 1500?
Your result has sense since there is no threshold for interior drianage basins. Here I am quoting the man page of GRASS 7.4 link:
The minimum size of drainage basins, defined by the threshold parameter, is only relevant for those watersheds with a single stream having at least the threshold of cells flowing into it. (These watersheds are called exterior basins.) Interior drainage basins contain stream segments below multiple tributaries. Interior drainage basins can be of any size because the length of an interior stream segment is determined by the distance between the tributaries flowing into it.
My guess is that you might get a better distribution of watershed with a lower value of the parameter threshold.