Hormonal

Resistance training alters the acute response of muscle protein synthesis (MPS) to exercise by attenuating the rise in non-myofibrillar (mixed) proteins while maintaining the synthesis of myofibrillar proteins, effectively refining the synthetic signal toward contractile proteins.

As you get stronger and more trained, your body becomes more efficient at building muscle. It stops wasting energy on non-essential cellular repairs (sarcoplasm) and focuses strictly on building the actual muscle fibers (myofibrils). This means you don't need to train to 'exhaustion' or do excessive volume to trigger growth; your body has refined its response to focus on what matters for size. Stick to progressive overload, but understand that 'more is better' has diminishing returns as you get trained.