Are these all OK?
It's more in demand.
It's in big demand.
There's a big demand for it.

And these both have the same meaning?
It's in big demand. / There's a big demand for it.

Question

Are these all OK?
It's more in demand.
It's in big demand.
There's a big demand for it.

And these both have the same meaning?
It's in big demand.  /  There's a big demand for it.

Answer

Yes they're correct but more commonly used is "It's in high demand".

All have the same meaning.