Among other things, the MacBook batteries are larger than the iPad batteries. Power consumption of the screen other components will obviously be different, but I would imagine that the MacBook Pro having a battery ~2x as large as that of the smaller iPad Pro would make a big difference.
M1 iPads have 28.65 Wh (11”) and 40.88 Wh (12.9”) batteries while the M1 MacBook Pro has a 58.2 Wh one.