Why 1.5 V batteries cylindrical and 9 V battery rectangle shaped?

For 1.5 V battery:

Batteries form a cylindrical shape to put the chemical electrolyte paste in layer around the center electrode. This is most efficient way of doing the things. If you roll a bandage over your hand it will be in cylindrical shape and the bandage is uniform around your hand. In similar way in cylindrical shape chemical get distributed around the electrode. So the reason is in front of you.

Now you may be confused why the 9 V batteries are rectangular right. That's really easy.

I think you know batteries are connected in series (one after another) the voltage gets added. That means if the three batteries are 1.5 volts each the net voltage will be (1.5+1.5+1.5)=4.5 V . But if you put the batteries in parallel then you will get 1.5 V. So, if you want make 9 V using 1.5 V you need 6  batteries connected in series.

If you open a 9 V battery carefully, you will see six batteries. All of which are of 1.5 V and cylindrical outer shape.
Following pictures speak themselves.

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