How come there are not too many of these types of blades? Why only defensive blades? Why not for offensive or all around players?

That is a very good question.

The offensive and allround blades generate power from the weight close to the handle, and thats where you have more control. When you hit further away from the handle, there will be more speed and less control. So, the offensive players don't need a larger sweet spot and the manufactures cut the part which does not help the purpose - it will save money also.

The players who loop as soon as the ball rises use very compact blades. The players who play further away from the table prefer regular size.

The defensive blades have less speed and more control, and that control is distributed throughout the blade because the weight is transferred throughout the blade. So, if the blade face has more space, it will have more sweet spot. Also, a semi-large or larger blades help defensive players to make less errors - when the blade has larger sweet spot the possibility of error is less.

Interesting comments.

If I may add: The offensive and allround blades used by the offensive players doesn't require a large sweet spot to hit the ball and wood being more expensive these days, the manufactures try to cut down their production cost.

I for one, uses a compact blades and played closer to the table. Other players who play further away from the table normally uses a regular size for speed.

I do agree with varghese comment that for a defensive player, they prefer large or semi-large blades which has a bigger sweet spot and more control.