@sunny malik
Which HBase version are you using? Default split policy for HBase 0.94 and above is not based on size. It is "IncreasingToUpperBoundRegionSplitPolicy". Assuming this is your split policy, and given your regions are less than 3.5GB, what you are seeing is expected behavior.
The default split policy for HBase 0.94 and trunk is IncreasingToUpperBoundRegionSplitPolicy, which does more aggressive splitting based on the number of regions hosted in the same region server. The split policy uses the max store file size based on Min (R^2 * “hbase.hregion.memstore.flush.size”, “hbase.hregion.max.filesize”), where R is the number of regions of the same table hosted on the same regionserver. So for example, with the default memstore flush size of 128MB and the default max store size of 10GB, the first region on the region server will be split just after the first flush at 128MB. As number of regions hosted in the region server increases, it will use increasing split sizes: 512MB, 1152MB, 2GB, 3.2GB, 4.6GB, 6.2GB, etc. After reaching 9 regions, the split size will go beyond the configured “hbase.hregion.max.filesize”, at which point, 10GB split size will be used from then on.
Please see the following link.
http://hortonworks.com/blog/apache-hbase-region-splitting-and-merging/
