TFA features a very reasonable explanation in the next few sentences. If participants are going to a session where they'll be essentially locked in with a psych

TFA features a very reasonable explanation in the next few sentences. If participants are going to a session where they'll be essentially locked in with a psychiatrist for a few hours and take only a placebo, they'll know it, and they'll get more depressed from knowing that they're in the placebo arm. So instead a small dose is provided which has some lesser effects, avoiding to break the participants' "blindness" to the protocol.