Your story can actually be compelling if you frame it around progression rather than trying to explain away the boutique move. Be clear about what you learned at BCG, why you chose the boutique, and what you gained there that you did not have before.

Then explain why returning to MBB now makes sense. The strongest version is usually something like broader responsibility or specialized exposure at the boutique, followed by a desire to return to larger-scale problems, teams, or development opportunities.

Avoid sounding as though the boutique was simply a mistake. The move should look intentional and additive.