Or HaChaim on Numbers 32:1/אור החיים על במדבר ל״ב:א׳

1 ומקנה רב היה לבני ראובן, And the members of the tribe of Reuven had a great amount of cattle, etc. The Torah had to give us this information in order to justify their request to settle in the lands which used to belong to Sichon and Og. The Torah simply testifies that their claim was based on solid facts. The reason that these two tribes had so much more cattle than the other tribes was that the soldiers of these tribes were exceedingly brave and captured more loot than the soldiers of the other tribes.