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The sizes of Gad and Reuben tribes were pretty average, however, Torah states that their livestock was disproportionally big (Num 32):
וּמִקְנֶה רַב הָיָה לִבְנֵי רְאוּבֵן וְלִבְנֵי־גָד עָצוּם מְאֹד׃
The Reubenites and the Gadites owned cattle in very great numbers.
What caused this fact?
Mechilta d'Rabbi Shimon bar Yochai, 12:38
ומקנה רב היה לבני (גד) [ראובן] ולבני (ראובן) [גד] יכול לבני ראובן ולבני גד היה לשאר שבטים לא היה? אלא מלמד שאלו פשטו ידיהם בו
How could it be that Reuven and Gad ended up with so much more cattle than the others? This teaches us that these two tribes stuck out their hands and [were quick] to grab [the spoils]
According to this understanding, it would seem that they were able keep whatever they grabbed before everyone else.
Reuben and Gad camped on the South side of the Mishkan with Shimon. The tribe of Shimon was decimated by the plague that God inflicted when people were sinning with the Midnianites. You can see the decline of their tribe between the census. Reuben and Gad must have taken the cattle after their Simeonite owners died.
They went from 59,300 to 22,200 people.
