Minimalism

Minimalism is the use of numbers that seem absurdly low for the situation/universe. This is a common brain bug, and one such example of minimalism run amok is is the so-called "3 million troopers" franchise decision in Star Wars. In a galaxy with at least a million worlds as part of the primary political entity, 3 million soldiers are entirely insufficient to fight a war on a pan-galactic scale. Other examples include the 200 vessel 'Katana' fleet in the Thrawn Trilogy, and the numbers provided for the majority of the battles during the Yuuzhan Vong invasion.

Why is this important?

While one could never call a space opera "realistic" certain things are expected. Tens of millions of men and women were mobilized during WW2, and no matter the level of technology, a certain number of "boots on the ground" would be required for any successful occupation.

Why do EU authors prefer minimalist stories?

In the decision to concentrate on a handful of characters in the SW franchise, most EU authors seem to have opted to minimize the importance of the greater universe. After all, with trillions of people out there, it's hard to argue that a smuggler and a farmboy (even if he is a Jedi) are the be-all, end-all to everything important that happens.

Alternatively, some authors may simply be unable to write to the numbers. Most EU material has been written during the immediate Post-Cold War Era of Operation Desert Storm and the operations in the Balkans, and the common layperson has come to equate high-tech militaries with a minimal amount of ground troops. Whatever their talents as writers may be, most EU authors are laypeople who very rarely, if ever, give much thought to how the military really works.

Another possibility is simple hack writing, with the author throwing in a tiny force out of laziness and declaring it a massive threat due to the perceived need to have everything occur on a galactic scale.