Sforno on Numbers 26:54/ספורנו על במדבר כ״ו:נ״ד

1לרב תרבה נחלתו, a reference to the amount of land, for although the land was being divided 12 tribes in shares of equal value in terms of money, the size of the allocations varied according to the quality of the soil each tribe received. A tribe who numbered many souls received a quantitatively larger piece of real estate as is spelled out here by the words לרב תרבו נחלתו, “you shall give a larger inheritance to the numerically superior tribe.”This is the reason why Menashe and Ephrayim received 2 separate territories seeing that their father Joseph had been a firstborn and treated as such in Yaakov’s final blessing, they were entitled to two such shares. (Genesis 48,22) This is also the way we must understand Chronicles I 5,1 ובחללו יצועי אביו ניתנה בכורתו ליוסף בן ישראל, “and when he defiled the bed of his father his birthright was given to Joseph, son of Yisrael.” Shimon, who numbered fewer people than any other tribe at this count, received a relatively small parcel of land, which itself was an enclave within the territory allocated to the tribe of Yehudah. (compare Joshua 19,9 on this subject) This also was the fulfillment of Yaakov’s wish in Genesis 49,7 “I will divide both Levi and Shimon among the tribes of Israel.”