Infinik“
Using the subject/object-case pronoun analysis, one can explain:
I like her as much as he [likes her].
I like her as much as [I like] him.
”
No, because
yours (and all other such pronouns) does not change from one case to another.
Yours is better. (subject)
Henry likes yours. (object)
Infinik “why it is grammatically incorrect?”
It means something absurd.
Compare:
My cats have the same weight as you.
= My cats have the same weight as the weight that you have.
= Both my cats and you weigh the same.
My cats have the same weight as yours.
= My cats have the same weight as the weight that your cats have.
= Both my cats and your cats weigh the same.
Thus:
I have the same book as yours.
= I have the same book as the book that your book has.
I have the same book as you.
= I have the same book as the book that you have.
I would not call as a relative pronoun. As far as I know it's a comparative conjunction, though it often has the appearance of a preposition.
CJ