Having worked there in the past this can be the view from the outside.

My anecdotal evidence is that those small starter teams do deliver alot of value, of which the client likes and wants more of. However the client does not have the resources to do this themselves (Likely the reason ACN are there to start).

Then from here it's self-fulfilling, more people leads to more projects leads more resources.

And usually, the client has money for TVCs but not enough internal resources due to dysfunctional management hierarchies.

Those same hierarchies will then inevitably fail to make proper use of the contractors, just as they failed to make use of their employees, leading to seemingly "useless" TVCs "bloating" the org. Symptom rather than cause of the client's failings.

The consulting playbook for these companies is bring in a rockstar team at or below cost for an initial project. Blow the client's mind with velocity and quality, then scope out of a bunch of follow on work using the highest margin, cheapest offshore resources they can find.