These are not circles!

It's like i said from the beginning Ellipses! :)

The more easy : make an Offset ineternat from the External Ellipse et voilà ! :)

Then move along the circular perimeter internal and you will have your same measures of your troncated volume wanted!

(troncated is not show here but very easy to make with all these advices :)

It's your choice after to want a circle inside but you will not have same distance between big & little section!

Here you have an ellipsoïd !