Hollw balls are made by casting the round part of the ball, and the internal part is also cast spherical.  The flow path is made by inserting a piece of piep (cylinder anside the ball and welding both ends at the opening.  the wall thickness of the cast part of the ball is usually thick enough since it must be thick enough to aloow the molten metal to flow though the mold. Check the weld size and wall thickness of the cylindrical pipe as well.  Use manual calculations, FEA, and TESTING to verify both the design and manufacturing process makes a useable part.
There's no theoretical reason you couldn't make a hollow valve ball.  If you're that severely weight- constrained, you might consider a butterfly valve instead.
Given that nobody stocks ball valves like that, it's custom, and the wall can be as thin as you want it. ... remembering that you'll be paying for machining time.
If you want to make it really thin, you need to consider the forces from the valve seats and the stem in addition to pressure considerations.  Some seats apply a lot of force and/or expect the ball to not deflect much.

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