SB 10.2.13

garbha-saṅkarṣaṇāt taṁ vai
 prāhuḥ saṅkarṣaṇaṁ bhuvi
rāmeti loka-ramaṇād
 balabhadraṁ balocchrayāt
Synonyms: 
garbha-saṅkarṣaṇāt — because He will be taken from the womb of Devakī to that of Rohiṇī; tam — Him (Rohiṇī-nandana, the son of Rohiṇī); vai — indeed; prāhuḥ — people will call; saṅkarṣaṇam — by the name Saṅkarṣaṇa; bhuvi — in the world; rāma iti — He will also be called Rāma; loka-ramaṇāt — because of His special mercy in enabling people in general to become devotees; balabhadram — He will also be called Balabhadra; bala-ucchrayāt — because of extensive bodily strength.
Translation: 
The son of Rohiṇī will also be celebrated as Saṅkarṣaṇa because of being sent from the womb of Devakī to the womb of Rohiṇī. He will be called Rāma because of His ability to please all the inhabitants of Gokula, and He will be known as Balabhadra because of His extensive physical strength.
Purport: 

These are some of the reasons why Balarāma is known as Saṅkarṣaṇa, Balarāma or sometimes Rāma. In the mahā-mantra — Hare Kṛṣṇa, Hare Kṛṣṇa, Kṛṣṇa Kṛṣṇa, Hare Hare/ Hare Rāma, Hare Rāma, Rāma Rāma, Hare Hare — people sometimes object when Rāma is accepted as Balarāma. But although devotees of Lord Rāma may object, they should know that there is no difference between Balarāma and Lord Rāma. Here Śrīmad-Bhāgavatam clearly states that Balarāma is also known as Rāma (rāmeti). Therefore, it is not artificial for us to speak of Lord Balarāma as Lord Rāma. Jayadeva Gosvāmī also speaks of three Rāmas: Paraśurāma, Raghupati Rāma and Balarāma. All of them are Rāmas.