Loki will be in the same group as Shirahoshi, Momo, and Conejo Blanco where they will represent their respective road poneglyphs and join as grand fleet members basically. So in a way Loki will "join" Luffy's crew but he's not gonna be leaving Elbaf on the Sunny.
It goes against the point of joining the crew if he's just chillin on a seperate ship. And to get meta his size would make it very hard to pair him with the crew in various marketing and promotional deals. He would stick out like a sore thumb, wouldn't fit with anyone, and throw the whole balance of proportions in the crew outta wack.
We've seen his individuality and personality in his flashback and it would make more sense to serve alongside Luffy as a leader of his own crew rather than under him.