Probably just fabricated stories for the press just to up their weight on the roster.
Unless they didn't lift in high school, you can maybe put on 1-2 lbs. a month over the course of a year if you do nothing but weight train and eat correctly and take a bunch of supplements. If you've been lifting for a while (i.e most high school athletes) its very hard to do that. The best you'll get is maybe 5 lbs. of muscle a year with dedicated training.
But basketball players who are 6'8" putting on 25 lbs. in 6 months? Impossible unless they're doing steroids and continuing to do nothing but train and eat.