You can apply the term 'boutique' as freely as 'small car'. If you are looking at strat houses MBB are not themselves boutiques, stand them next to Accenture or IBM and they suddenly look rather like boutiques.
The problem is that you can freely apply this term to tier 2 (so called) strat firms like LEK, Monitor, Wyman, Roland Berger etc and none are actually 'boutiques'. A real boutique in the strict sense would be small operations with a strict definition of their expertise - and they are hard to identify - so I would point to firms like Cairneagle or Cairnforth, State of Flux, Efficio, Inzenka, Capco etc as being 'boutiques' at the larger end of the scale.
Essentially you will hear this term bandied about but it really only applies - technically - to firm which are first and foremost 'lifestyle' businesses, which none of the above are - or small organisations (which many are).
I.e. is the new BMW Mini a small car? Looks larger than the old mini, bigger than a Fiat 500, but smaller than a BMW 3 series (and is THAT a small car? Compare to 5 series? ad nauseum).