I am not a topography expert, but I believe one of the prime factors for deciding the contour interval is the scale of the map. A smaller scale map will have lower number of contours (to avoid cluttering), thus the contour interval will be higher between two contour lines. A larger scale map can have higher number of contour lines (also depends on the topography/relief of the terrain), thus the contour interval can be smaller between two lines.
e.g. I can see some topographical maps of Cambodia (Scale 1:50,000) where contour interval is 20 meters.
*The organization (usually the National Mapping Agency) that prepares the topographical maps, decides what is the best suited contour interval for a map at a particular scale.
Think Location
