Looking at NIMH data[0], about 7.1% of US adults are diagnosed with depression, and about half of those diagnosed receive medical treatment - so at least 3.5% of the population is receiving medically prescribed treatments that are known to cause weight issues from depression alone. Adding other mental illnesses, endocrine disorders, and drug abuse, I would bet that at least 10% of the population is receiving drugs that are known to affect weight regulation mechanisms.
Still, this is a far cry from the >30% experiencing obesity, especially since not all psychoactive drugs alter body weight, and only a percentage of people taking those drugs that do will experience weight changes.
That aside, yes, the majority of overweight/obesity is a consumption + lifestyle choice.