David Henderson had an admirably empirical response to Bryan Caplan’s proposal that prisoners be segregated by weight. I figured there should be data for this and set about googling. I found Prison Victimization: High-Risk Characteristics and Prevention by Leslie A. Swales of Kent State which says the following on page 27:
The average weight of a sexual assault victim in prison is 141 pounds. This is lighter than the average offender. The offender also stands one inch taller than his target (Man & Cronan, 2001).

UPDATE:
Bryan linked to an older post of his which referenced a study supporting his assertion about the size of victims in prison. In the comments Levi gave a bunch of stats about the rate of sexual assault at Ivy League schools:
Brown University, 8,020 students, 3 forcible sex offenses
Columbia University, 24,820, 9 forcible sex offenses
Cornell University, 19,800 students, 3 forcible sex offenses
Dartmouth College, 4,147 students, 6 forcible sex offenses
Harvard University, 19,140 students, ~17 forcible sex offenses
Princeton University, 4,918 students. ~12 forcible sex offenses
University of Pennsylvania, 19,816 students, 12 forcible sex offenses
Yale University, [11,593] students, 9 forcible sex offenses

Levi gave just 11 students at Yale, I replaced it with the total fall 2009 enrollment from here.

UPDATE 2: Henderson asked to know how large the weight-difference was between victims and offenders. The paper linked above cites Man & Cronan Forecasting Sexual Abuse in Prison: The Prison Subculture of Masculinity as a Backdrop for” Deliberate Indifference”. That paper in turn cites Alan J. Davis’ “Sexual Assaults in the Philadelphia Prison System and Sheriff’s Vans” as saying the average weight of the rapist was 157 pounds, compared to 141 for victims. Man & Cronan also cite Daniel Lockwood’s “Prison Sexual Violence” as saying victims average 15 pounds fewer than their aggressors.