+1 Support
First Reason: Defense. Shock are used to defending the brig from mingy CTs, and prisoners are often brought by there for detainment anyways. This would both give Shock more to do, and logically makes lore sense. If moved to SOBDE Bunks... Its SOBDE. That's all that needs explained.
Second Reason: Convenience. Since Shock/SOBDE respawns there, it means even in the event of attack, Shock/SOBDE can very quickly report to defend again.
If ICU was in SOBDE Bunks for some god awful reason, it wouldn't be Medic's call to move it because they are the ones to use it, it would be SOBDE's because it is in their space. The flip side can be said for IR in Medbay. It is the Medical team's call first, because it is their space.
First off, that is very selfish of you, considering Shock is the only battalion (save for maybe SOBDE, IDK cause I have never seen their bunks) to have massive spaces and defenses like the Detention Level.
Secondly, adding a room in the map is far more difficult than simply moving some props and a textscreen onto a pre-existing room, and may even cause more optimization issues if a new room is added.
Considering that the larger cells in the brig are larger than the cell in Medbay used for IR (and the smaller cells are very close), there is no reason that the brig one would be "too small". To further that point, there is only ever a need for space for two individuals inside the IR: the prisoner and the interrogator. If anything, a smaller space could be used as a "enclosement" tactic, leaving the prisoner feeling confined and trapped.