Solder Powder in Flux
The mass of the powder (Sn63/Pb37) is 8.5 grams per cc, the flux is about 1gram per cc.
The normal loading for printable paste is 90% (by weight) of powder. The dispensable grade has a lower viscosity the lower metal loading - about 86%.
By volume the story is very different. Solder powder (Sn63/Pb37) takes 1/8.5th the volume the flux takes. The flux is between 10-14% by weight. The volume is about half flux and half metal.
SAC305 density is 7.37 grams per cc. The loading needs to be less to get to the same volume ratios.
This matters most in dispensing - forcing a 90% loaded solder paste down a needle is pretty close to impossible. The reduced loading increases the flux, decreases the viscosity and allows the solder paste to flow. Solder paste, being a highly viscous material, takes a lot of energy to force through the needles. This can be helped with a conical nozzle.