Certainly geld any horse/colt, which isn't or should not, be breeding quality. We try to geld when flies are not a problem and geld all colts before sale, unless being run on as possible breeding quality. Way too many whole colts and stallions out there, being sold whole, when there is a huge possibility they will be bred in the future and not offer anything to the breed.
It is true, that colts (and male dogs) gelded/neutered very early, might grow a wee bit taller than those left and gelded at maturity. Tis was proven by Mastiff breeders some year ago. If you don't want a Mini to go over-size, but intend to geld in the future, then it is up to the owner to make sure he is kept within stallion fencing until the 'event' and not breed any mares.
Lizzie