In EVM the caller must define the available space for return values.

In the current EVM2 design this is carried over, however it could be improved:

• call doesn't defines the return value space
• return doesn't writes the value to the caller's memory space (I understand this today can depend on the VM anyway)
• return places the values into an intermediate, in-memory storage (callstore) and the contract is charged according to the size of this
• a new opcode, callResultCopy is introduced for copying between callstore and the caller's memory space (and callResultSize to retrieve the total size)
• the callstore is erased when a call is executed

It is a rough design, but it could be ironed out.