1. The man is twice as tall as Bill.

Question

2. The man is twice taller than Bill.
(Are both the same and grammatical?)

3. This T-shirt costs twice as much as that one.
4. This T-shirt costs twice more than that one.
(Are both the same and grammatical?)

Answer 1

1 and 3 are correct, but 2 and 4 are not.

There's no explanation. It's simply normal idiomatic expression.