I looked back at the UConn players with similar body types as Johnson. It's typical for a player to gain 20 to 25 lbs during their career and as of now (based on roster listing), Johnson has gained 20 lbs which is within the norm. You have to remember, Johnson was brought to UConn as a stretch 4, not as an inside banger as he doesn't have banger body type.
Some UConn weight gains:
Johnson: 205 to 225 lbs.
Brimah: 217 to 232 lbs.
Akok: 193 to 225 lbs (this year at WVU)\
Armstrong: 210 to 235 lbs.
Whaley: 190 to 225 lbs.
Facey: 200 to 227 lbs.
Nolan: 203 to 230 lbs.
Daniels: 195 to 195 lbs.