Śrīla Viśvanātha Cakravartī Ṭhākura has commented on this verse as follows. When Lord Kṛṣṇa was of a very tender age, His hands and legs resembled soft new leaves, yet simply by touching the handcart with His legs, He made the cart fall to pieces. It was quite possible for Him to act in this way and yet not exert Himself very much. The Lord in His Vāmana avatāra had to extend His foot to the greatest height to penetrate the covering of the universe, and when the Lord killed the gigantic demon Hiraṇyakaśipu, He had to assume the special bodily feature of Nṛsiṁhadeva. But in His Kṛṣṇa avatāra, the Lord did not need to exert such energy. Therefore, kṛṣṇas tu bhagavān svayam: (SB 1.3.28) Kṛṣṇa is the Supreme Personality of Godhead Himself. In other incarnations, the Lord had to exert some energy according to the time and circumstances, but in this form He exhibited unlimited potency. Thus the handcart collapsed, its joints broken, and all the metal pots and utensils scattered.
The Vaiṣṇava-toṣaṇī remarks that although the handcart was higher than the child, the child could easily touch the wheel of the cart, and this was sufficient to send the demon down to the earth. The Lord simultaneously pushed the demon to the earth and superficially broke the handcart.