tapered extrusion

Here I took a totally different approach. I did it manually.

Star Tool, Place a star. Free Transform.
Turn on Smart Guides. Snap to point
Line Tool, draw some lines.
Direct Select each end point, move to star.
Bottom end points, Object>Path>Average. (This gives you the Vanishing Point.)

Live Paint. Fills. Recolor as you wish.
Expand Live Paint, Delete strokes. Select same color and Pathfinder>Unite.
Perhaps play with gradients.

K

You can use perspective during creating your 3D as Ton says or create a Blend object with a Smooth Color option from large to small stars. The last way allows you to control the extrusion depth easily and change the sizes of starting and final objects at any moment.

If you want to do it manually, you could use the Classic 3D and expand appearance.

The new 3D and Materials has an option in the latest AI update to expand as Wireframes (to be found at the bottom of the Object section).

Both the new and the classic 3D effects have a perspective option to use with a large extrusion.

I'm not sure what kind of object you are talking about, cann you please show a screenshot?

If this is 3D, please tell us exactly which methods you applied.