The only comments I've seen regarding the size of people riding ponies have been directed towards people obviously far to big for the pony.
I have no problem with adults riding ponies, in fact I 100% agree that a younger pony needs a smaller adult/teen to hop on every now and then for a "tune up". I'm only 5'1 so was often handed that job through the years.
Stop for a minute, open your mind, learn. You may not agree with what I say, I may not agree with what you say but we will both learn something new.