If your mind was permanently stuck at an infant's cognitive level, you wouldn't care what size you were or why. However, this child sounds likely to be able to perceive the difference between being cared for by loving parents and being put in an institution to be cared for by a revolving cast of shift-workers. That was going to be the choice if she grew to adult size. Aging parents aren't necessarily able to lift a 150 pound person in and out of a super-crib, in and out of the bath, in and out of a car, etc.
Keeping severely mentally impaired children from reaching full size isn't necessarily just a matter of "convenience", it can be a matter of safety. When children who are prone to physical aggression or uncontrolled movements reach adult or near-adult size, they can often pose a serious danger to their parents, siblings, and non-family caregivers.
One woman who used to post on FR, and has a mildly autistic child of her own, told of a friend of hers who was determined not to put her severely mentally disabled (autistic) son in an institution. Among other things, this involved locking him in a padded room in the attic every night, both for his own safety and that of other family members. Still, when he reached his teens, the extreme measures which had defined the household for years weren't enough anymore. Eventually they institutionalized him, but it was only after one of his unpredictable bouts of violence had inflicted permanent vision damage on a younger sibling.
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You make some good points. I thank God I have not had to make the kind of decisions these parents have.