accretion discs - Minimum Mass Solar Nebulae Scaling Factors

Attempt to do an intuitive explanation:

The Sun was formed from a cloud of matter. A small amount of that matter remained around the Sun, and had a similar composition. This was the solar nebulae.

But the composition of the planets are now not the same as the Sun. This is explained by that some of the matter was collected by the Sun, and some of it was blown out of the system.

That was only the light stuff! like hydrogen and helium. Most iron and other metals are conjectured to have remained in orbit, forming the planets.

As the Sun is representative for the original composition (partly at least, stellar fusion has skewed the hydrogen/helium ratio). So if iron is 0.14% of the Sun, that is also the original abundance in the nebulae. So all the iron in the planetary bodies is 0.14% of the original nebulae mass.

From that you can calculate the original mass. It is a 'minimum' because some of the iron may have escaped as well.