They both look like they might be similar sizes in an illustration like that, but the Hubble mirror is 2.4 meters, whereas the JWST mirror is 6.5 meters, which

They both look like they might be similar sizes in an illustration like that, but the Hubble mirror is 2.4 meters, whereas the JWST mirror is 6.5 meters, which has 7 times the light-gathering capability of Hubble.