Even though as already said you should get some more power standing you’re also using more energy and the heavier you are the more weight you’re putting on your leg muscles....and then i also have a friend I ride with occasionally who has such a large strength to weight ratio that she can do long climbs entirely off of the saddle.

Perhaps a better body position and rhythm will help your power output. If your bars are too low it will make getting an optimal position difficult. You may also just need to practice it to get your muscles to adapt to the position for both motor learning and muscle development reasons.

I ride long steep climbs on my weekend rides for my strength and endurance training. I’ll get off the saddle when I need some power to get around someone etc., to stretch my back or on a really difficult climb to make changes in my muscle groups. For a smooth transition I’ll move my hands to the hoods, shift and stand as my right foot begins its downward stroke. I’ve learned by watching the movement of my stem I can quickly get a fluid rocking movement and know the rhythm that allows my leg to make a direct push on the pedal making the best use of my weight and keeping a smooth stroke. Keeping my chest open for breathing efficiency, not leaning forward enough to put weight on the front wheel but enough to put my weight over the axis of the crank rotation and having the tip my saddle just barely brushing my thigh gives me a body position that works well for me. Either sitting or standing I try to keep a smooth pedal stroke and my body relaxed and breathing efficiently.