I’ve learned that I’m not the only person to propose weight classes for prisonsHere‘s Philip Ellenbogen in the Columbia Journal of Law and Social Problems:

As discussed earlier in this section, a victim of prison rape is often much smaller and weaker than the perpetrator of the violence.  Therefore, while prior violence should certainly be taken into account, the classification system should incorporate the strength and size of the inmate as well. This Note encourages prison officials to continue to use degree of violence as a factor but within that violence classification, to check each inmate for height and weight, and classify each according to a sliding scale, the details of which will be discussed later in this section.

See pp.363-8 in the article for the full discussion.