I'd add that IME lighter riders are usually better out of the saddle than heavier riders. It doesn't take leg strength to turn the pedals when up, it takes leg endurance. This can only be cultivated by spending a lot of time up. It's a lot easier to turn the pedals standing on a fixie than geared, so just do it a lot. It takes time to get this sort of endurance, though. Doesn't come overnight.

You can also do the opposite: learn to grind better. I recommend spending time on the trainer doing one legged intervals on a freewheel bike, keeping the chain tight. Then on the fixie, work on what you've learned about keeping a leg on it all the way around, not accelerating on the downstroke. Grinding is about increasing the strength of little-used muscles, and of course also their endurance. What I'm saying is learn to pedal slower when grinding by powering all the way around. 30 rpm is completely reasonable. Just slow it down, keep your upper body still, and keep pressure on the pedals.

So try standing on the shorter hills, like 10 minuters, and grind up the passes with out of the saddle breaks.