According to the book of Numbers, the size of the inheritances of the tribes of Israel depended on their fruitfulness and their faithfulness.
After the second census (Num 26), the LORD tells Moses that the size of each tribe's inheritance shall be "according to the number of names" (26:53). A large tribe gets a large inheritance; a small tribe gets a small inheritance. Two things determine the size of a tribe at this point.
(1) Fruitfulness. Fairly obviously, a tribe whom the LORD has blessed with larger numbers of offspring in the preceding decades will have more names on the list, and thus a larger allotment of land.
(2) Faithfulness. Look back at the numbers of people in each tribe. That's right, the long, boring bits that you skipped over. And compare the sizes of the tribes in Numbers 26 with the sized in the first census in chapter 1. What do you notice? That's right, they're all pretty much the same, and indeed some have increased a little, with one glaring exception: the tribe of Simeon has decreased from a mighty 59,300 to a lowly 22,200 (1:23; 26:14). How did all those Simeonites die? That answer is in the previous chapter, when following the Baal of Peor episode, the LORD struck Israel by a plague, killing 24,000 people (25:9). And which tribe suffered most in the plague? Which tribe lost a vast proportion of its members, and thus was destined to receive a correspondingly smaller inheritance? Look no further than the identity of the man slain by Phinehas the son of Eleazar (25:14): "Zimri, the son of Salu, chief of a father's house belonging to the Simeonites."