At standard conditions, a single mole of oxygen occupies 22.4L. The conversion rate from NaClO3 to O2 is 2-to-3, so for every two moles of sodium chlorate you burn, you get three moles of Oxygen, or 67.2L.
The molar mass of NaClO3 is 106.44g.
So some quick math, to fill 2600L we need 116 moles of oxygen, which we can produce with 77 moles of sodium chlorate. Multiply by the molar mass and we get ~8kg.
Sodium chlorate has a density of 2.49 g/cm^3, so an 8kg slug will fit in a 3.2L bucket.
What this boils down to is "salt is a couple orders of magnitude more dense than air, so we can fit a lot of air in very little salt". Which is a sentiment I think most people can feel is correct even if they don't know their ideal gas law or whatever.
The molar mass of NaClO3 is 106.44g.
So some quick math, to fill 2600L we need 116 moles of oxygen, which we can produce with 77 moles of sodium chlorate. Multiply by the molar mass and we get ~8kg.
Sodium chlorate has a density of 2.49 g/cm^3, so an 8kg slug will fit in a 3.2L bucket.
3L of powder gets you 2600L of oxygen, not bad!