eka-i svarupa donhe, bhinna-matra kaya
adya kaya-vyuha, krsna-lilara sahaya
eka-i—one; svarupa—identity; donhe—both of Them; bhinna-matra kaya—only two different bodies; adya—original; kaya-vyuha—quadruple expansions; krsna-lilara—in the pastimes of Lord Krsna; sahaya—assistance.
They are both one and the same identity. They differ only in form. He is the first bodily expansion of Krsna, and He assists in Lord Krsna's transcendental pastimes.
Balarama is a svamsa expansion of the Lord, and therefore there is no difference in potency between Krsna and Balarama. The only difference is in Their bodily structure. As the first expansion of Godhead, Balarama is the chief Deity among the first quadruple forms, and He is the foremost assistant of Sri Krsna in His transcendental activities.

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