Size is a means of accomplishing greater physical feats, but if you don't have the sense to utilize these tools you struggle. IQ in Marners case allows him as a player to utilize his undersized frame to the best of his abilities. In a larger frame, in unison with having a strong work-ethic, drive and high IQ it isn't a large leap of faith to assume he puts in the extra training. His skating technique is great, in a larger frame he then needs the additional muscle to move the additional mass to reach par for him, he already has a good two-way game and with added size would be more of a force. He would be a better overall player.
In this odd hypothetical then yes, adding a few inches on Marner would inevitably lead to a superior player. Adding 20 IQ points to Strome would not make him a better players because his major flaw is getting from point A to point B, the additional IQ points might make him realize he needs help with his skating. But in either scenario Marner is only behind McDavid is pure skill.