Do you stander-uppers use the same gear standing as sitting, or a differnet one? (Obviously if you decided to stand up as the hill got steeper rather than change down, that is not the same gear.) I find using a harder gear makes standing up pedalling easier for me. Attempt to explain this below…
I am quite old and not a very strong pedaller. I find that if I stand, my natural reaction is to ride harder. Unless I am in a harder gear than I would be sitting, I rapidly get out of breath. I put this down to the natural amount of pedal force that standing up induces (very roughly* one’s body weight on the pedal). In a given gear, this implies a certain power output. In my sitting gear, for me this is a bit above my maximum sustainable. In a cog harder, the cadence slows, reducing the power output for the pedal force, and I can keep it up (oo er missus). I am guessing that a younger/fitter rider would not experience this so much (I can’t actually remember how it felt when I was younger). Overall, I prefer to sit when I have gears, standing up only in short bursts, or when it gets so steep I have run out of gears on the road bike. I just stay seated and mash the pedals harder on the mtbs, hardtail and full suss.
I used to single speed a lot, and my commuter is a single speed (though I no longer commute). I can still do long climbs single speed standing up (only on the road bike, I have given up off-road single speeding), but of course that is in a much higher gear than I would be in sitting on my geared bike, so it supports this theory. I think that I do sit more single speeding than I used to though.
* of course you can vary this by taking more body weight on the handlebars for an easy ride, leaning back and pulling on the handlebar to generate more force.