And how you make 4 MB out of 1 MB? You have 32 bytes for Segwit commitment in the coinbase transaction, and this is all you need, to go from 1 MB legacy space to 4 MB witness space. Here, it is similar: you have 32 bytes for R-value of ECDSA signature, and you can store everything behind it, including a quantum resistant 50 kB signature, for each and every R-value you currently have, when OP_CHECKSIG or its equivalent is called in the Script.

Of course, all old nodes won't notice block size increase, in the same way, as non-Segwit nodes still think, that the maximum block size is 1 MB.