Tatum will naturally gain as he gets into his 20s. At the predraft he was only 204lbs. I suspect he is already heavier than that. I suspect in his prime his ideal playing weight will be around 225-230lbs. The league is full of skinny underdeveloped players who gain 15-25 lbs from their predraft weights till they hit their primes. A good example is to compare rookie photos to current photos of players like Hayward, Anthony Davis, and Durant. Jaylen Brown was 223 and a good sized wing at the draft and was reported to be up to 237 this off-season. I suspect a similar gain from Tatum this off-season. This would put Tatum around 220lbs not to mention the rumors that he has grown is in 6'9 rather then his listed 6'8.
Tatum's weight/strength gain is going to be very important to the Cs future. It will hold the key to the Cs being able to defend while playing the impressive offensive of Irving,Brown,Hayward, Tatum and Horford next season.