We say that two languages $L_1,L_2$ are decreasing reducible if there exists a polynomial time reduction $f:\Sigma^*\to\Sigma^* $ and there exists $n\in\mathbb{N}$ such that for every $x\in\Sigma^*$ satisfying $|x|\ge n \implies |f(x)|\lt |x|$.
Assuming $P\ne NP$
Prove\Disprove: Every two NP-complete languages $L_1,L_2$ are decreasing reducible.
I'd appreciate a hint or direction