In 1.8.1, a single block of TNT would blast an area about 3x3x4, but since the update to 1.8.2 and above, the blast radius is slightly smaller. The attached screen shots show a test using the exact same section of rock in 1.8.1. and 1.8.3. I dug a hole four blocks deep and placed one block of TNT at the end.

After-TNT-1.8.3.jpeg shows fewer damaged blocks on the side closest to me than in After-TNT-1.8.1.jpeg, including two complete rows of block on the floor.

(I've seen reports similar to this affecting earlier Minecraft versions, but I'm not sure if this is a new issue or a continuation of those earlier ones. Moderators, please move if necessary)