you mention bud rot in diary... you can't force a plant to do much of anything. The infection could be source of the slowdown in growth too, but more likely just genetic diversity..
don't start flush if u want that one to get bigger.. keep eye out for more rot, as it probably is not gone. weigh rate of decay vs. how much time you need etc.. if the rot is at bay, then you might be able to let it fatten up with time.
that plant could just be different genetically... not all plants can produce donkeydick colas. Not all sister-seeds will have the same flower time.