The Big 3 distinction isn't based on firm size (McKinsey, Bain, and BCG only have 38k, 25k, and 15k employees respectively) but rather on prestige and type of work done.
Dell is not lumped in with MAGMA even though they’re everywhere and everyone knows who they are because they have completely different business models, employee pools and compensation. They’re barely in the same industry, if you squint. Similarly with Accenture and prestige consultancies. If you were going to have a Big 4 the fourth would be AT Kearney, not Accenture.