Changing characteristics of precipitation extremes have been reported mainly using parameters like the frequency of occurrence and the magnitude. As climate changes, the spatio-temporal characteristics of rainfall extremes are also likely to get modified. Though there exist studies noting the changing the temporal distribution, the spatial extent of extreme rainfall events has received less attention. We show that 31% of the fractional increase in the number of rainfall extremes of the Indian summer monsoon from 1951 to 2015 is size-change related. We find that the average size of rainfall extreme is significantly increasing after 1980. Depending on the number of connected grids experiencing simultaneous extreme rainfall, we classify them as small, medium and large events. 90 % of large events have occurred after 1980 and these have the strongest rainfall intensity among all types. They are more likely to cause floods, hence important. These events are invariably associated with a monsoon low-pressure system (LPS) and occur in the southwest sector of an LPS within 400 km from its centre. Strong surface pressure anomalies that persist for more than a week are present in different parts of the globe when these events occur, suggesting preferred planetary-scale conditions that favour their formation. Thus giving hopes of their advanced prediction and facilitating flood hazard mitigation.