John Gill Commentary Numbers 32:1

"Now the children of Reuben and the children of Gad had a very great multitude of cattle: and when they saw the land of Jazer, and the land of Gilead, that, behold, the place was a place for cattle;" — Numbers 32:1 (ASV)

Now the children of Reuben and the children of Gad had a very
great multitude of cattle
By which it seems that they had more in proportion than any of the other tribes; by what means it is not easy to say; very probably they were more addicted to the pastoral life, and took more delight in breeding cattle, and were more diligent in taking care of them: however, the reason for this observation presently follows:

and when they saw the land of Jazer, and the land of Gilead ;
Jazer was in the kingdom of Sihon, and Gilead in the kingdom of Og, which had been both conquered by the Israelites:

that, behold, the place was a place for cattle ;
where much cattle was fed, there being a great deal of good pasturage for them: Jazer appears to be a well watered country, (Jeremiah 48:32) , and Gilead and Bashan which joined and belonged to the same country of Og, who was king of Bashan, were famous for good feeding of cattle: hence we read of the bulls of Bashan, and rams of the breed of Bashan, and goats on Mount Gilead that looked plump and sleek; see (Micah 7:14) (Deuteronomy 32:14) (Psalms 22:12) (Song of Solomon 4:1) .