In Bitcoin proof-of-work, the difficulty of Proof-of-Work is estimated and calculated based on total hashing power of the participants. If total hashing power of the participants is higher, then PoW is more difficult.

• If we use a Memory-Hard-Proof-of-Work scheme, the difficulty of Proof-of-Work is still estimated based on the hashing power ? OR based on the size of memory ?

• In other word, is it feasible to design a Memory-Hard-Proof-of-Work scheme in which we could estimate the Pow difficulty based on the size of memory, instead of hashing power of participants ?

• Has such a Memory-Hard-Proof-of-Work scheme designed yet by which we could estimate the difficulty based on memory size?

Just as an example: a Memory-Hardened Proof-of-Work Scheme: https://eprint.iacr.org/2017/1168.pdf

P.S. 1: Related question: https://crypto.stackexchange.com/q/29890/68724

P.S. 2: If you need any further complementary explanation about the question, please let me know.