According to Diogenes Laërtius, the philosopher was named Aristocles (Ἀριστοκλῆς) after his grandfather. ... The sources of Diogenes account for this fact by claiming that his wrestling coach, Ariston of Argos, dubbed him Platon, meaning 'broad,' on account of his robust figure.
Seneca said, "[Plato] was the fortunate possessor of a strong and sound body (his very name was given him because of his broad chest)..."
That's right.
"Plato" was his wrestling stage name.