CC Ādi 5.5
আদ্য কায়ব্যূহ, কৃষ্ণলীলার সহায় ॥ ৫ ॥
ādya kāya-vyūha, kṛṣṇa-līlāra sahāya
These two are one and the same identity. They differ only in form. Lord Balarāma is the first bodily expansion of Kṛṣṇa, and He assists in Lord Kṛṣṇa’s transcendental pastimes.
Balarāma is a svāṁśa expansion of the Lord, and therefore there is no difference in potency between Kṛṣṇa and Balarāma. The only difference is in Their bodily structure. As the first expansion of Godhead, Balarāma is the chief Deity among the first quadruple forms, and He is the foremost assistant of Śrī Kṛṣṇa in His transcendental activities.