I'm moderately heavy, for a cyclist anyway... 5'7", 165-170 lbs (~12 stone ). I stand a lot when I climb, and practice it, trying to improve and to ensure standing doesn't become a weakness. In spite of that I find that I can climb faster, longer seated, and when standing, I need to carry a fair amount of weight with my arms, so that I can spin. FWIW, I'm talking about 6+% grades, here. (Standing w/out a steep hill is a whole different kettle of fish, and not particularly difficult.)

So anyhow, sometimes I stand a little to give my legs a break on long climbs, or when going over a short roller, but for general climbing it just doesn't work for me. Is anyone else similar? I'd be curious to hear comments.

And, yes, focusing on pedaling circles makes a huge difference. If you climb by just pushing the pedals down, you'd be surprised how much energy you waste trying to make the pedals go where they can't! Mascardr, being a lightweight, you're probably better off developing your standing technique, and as others have said, develop strength/power. Just a guess... good luck!