Rock of Ages on Broadway plays at the Helen Hayes which is a similar size to the Duchess so it could have downsized for that theatre. They probably have to downsize anyway for the Garrick unless the current set can fit.
The Helen Hayes has a larger auditorium (60 more seats) and stage than the Duchess. When you talk of size; do you mean bums on seats or stage size?
I would think a major issue would be reduced pay for the Company as the Duchess or Garrick would send the Equity grades down a notch?