I found that for myself, bulking up too much offseason didnt translate to onstage weight
(At least in condition). Instead of bulking excessively I kept my diet a bit cleaner very high protein and gradually increased dose aas through dieting phase. Was able to gain muscle while prepping up until maybe a month. I know this is different than the initial 15 years spent gaining the base size though.
As you age, the muscle gains from a gross off-season period tend to decrease, it is also more difficult to eliminate excess fat when you go to the pre-contest diet, so more mature athletes tend to stay cleaner throughout year.