2” hose creates less turbulent airflow (laminar airflow)than 1 1/2” hose which creates more turbulent airflow so it’s more efficient. The longer the hose run the more noticeable the difference is. When using a 25’ length of hose you may not notice a difference but over 50’ or more you certainly would. In simple terms I think it may be described as friction between the moving air and the inside of the hose and the 1 1/2 “ hose creates more ‘friction’ so looses more energy than the 2”. Hope this helps.
Nobby