Balthazar is a reference to a historical figure, one of the three wise men who brought gifts to the newborn Jesus. However, the internet quickly adapted this term to represent someone who is perceived as exceptional or larger than life.
When coupled with XXL, the term connotes an exaggerated sense of grandeur. Balthazar XXL essentially became a title for Baltasar Engonga, transforming him from a regular internet personality into a much larger figure in the eyes of his fans.
The "XXL" tag has been a common way for internet culture to exaggerate and amplify individuals, turning them into legends of sorts—often humorously. For Engonga, it's become part of his viral identity.