For future reference, the exact Wikipedia quote is: "Their numerical dominance, often exceeding a million individuals per square meter and accounting for about 80% of all individual animals on earth, their diversity of lifecycles, and their presence at various trophic levels point to an important role in many ecosystems."
I could not look at a free online copy of either source, but I have a guess where the 80% figure comes from. Imagine giving every living individual animal a number and then holding a lottery where every number you gave out has an equal chance of being picked. What I believe this quote is saying is that there is an 80% chance that the winner of the lottery will be a nematode.
The reason so many individual can live is so small a space is because most species are extremely small. This site offers data on the average weight of individuals in each nematode families and the largest was steinernematidae at less than 115 micrograms. Wolfram Alpha says that is less than half the mass of a grain of sand.