I'm not going to argue that Smith's game is going to translate - jury still out for me. But these reasons don't square.
Size: He's 18 years old and can't yet grow a beard. Very likely he has a little height left to grow and definitely has weight to add. He's 6'0" where average centers are just under 6'1". He's probably 185, so skinny, but again he will be gaining muscle until he's 22-23 at least. Size isn't the reason.
Speed: he has quick bursts and he has shown decent top end speed even as recently as WJC. issue is it's inconsistent. That's either a style problem (too passive and waiting for the play to develop, or not enough compete/intensity), an endurance problem (often looks clearly slower at mid/end of shift), or a burst problem (not high end enough fast twitch).
All of those can improve, but how much will they improve? It's simplistic to say he's "a slow skater" when he has shown anything but, but only at times.
On compete, he certainly comes across in all interviews as hyper competitive, but his style on the ice is less "dog on a bone" like Leonard and more "sit back like the hawk and wait." That may need to change or he may just need to be slightly more tenacious at times.
Is he going to be able to adapt his style and learn quickly? There's still a lot of time. But the variability of performance is more concerning to me than "size or speed."