'Direct' translation

Suppose we have two trees. They are exactly the same size.

This tree is as big as the other tree.

Cet arbre est aussi grand que l'autre.

Supposing one is larger than the other…

This tree is bigger than the other tree

Cet arbre est plus grand que l'autre

Supposing one is 100% larger than the other. It's 200% of the first tree's size.

This tree is twice the size of the other tree
This tree is twice as big as the other tree

but…
Cet arbre est deux fois plus grand que l'autre

which is like saying 'two times bigger' and suggests that the tree is not 200% but 300% of the original size…doesn't it?

French has a strange sort of logic :-)