Broly's body "exploded" in a metaphorical sense because it was utterly overwhelmed by an uncontainable surge of energy during his transformation, a state brought on by his already maximized power combined with a sudden, immense boost from Goku.
The Overload Phenomenon
At the time of his climactic confrontation, Broly had already reached his absolute peak power. His immense strength and destructive potential were already pushed to their limits, indicating that his physical form was barely able to contain his inherent energy. This pre-existing state of being at full power made him incredibly susceptible to any further increase in energy.
The Catalytic Boost from Goku
The situation escalated dramatically when Goku, in a desperate attempt to defeat the Legendary Super Saiyan, transferred his energy to Broly. This was not a controlled absorption or a gradual power-up; rather, it was a sudden and massive influx of power into a being whose internal energy reserves were already beyond capacity. Broly's body, already straining, simply could not accommodate this additional, colossal amount of energy.
A Metaphorical Implosion of Power
Consequently, the sheer volume of power became too great for Broly's physical form to contain. It was a rapid, internal breakdown where the overwhelming energy had nowhere to go, leading to a catastrophic expulsion. While not a literal physical explosion in the conventional sense, his body was unable to sustain its integrity under such immense pressure, effectively resulting in an "explosion" of his being as the excess power tore him apart. This event symbolized the ultimate consequence of power without control, leading to self-destruction.
