On a regular blockchain, an open transaction is stored compactly because everything is visible directly.

On a secure blockchain like Zcash, an encrypted transaction takes up more space:

it needs to hide data under a layer of randomness built through polynomials and cryptographic proofs.

If a quantum computer appears, then theoretically it will be possible to attack some old schemes (especially ECDSA).

Halo 2 builds proofs only through the algebraic properties of groups and polynomials,

without fixing a secret section at the beginning.