At some point, the tech-priests of Mars captured the Doctor in a stasis field and pulled apart the T.A.R.D.I.S.
Ever since, all STC-based vehicles have been far bigger on the inside. Most Rhinos contain several rooms, including
a pool, gym, and lounge with easychairs.
If you're gonna fight the enemies of Mankind, you may as well do it in comfort.
And remember, carnifexes are the kind of thing that can take a krak missile or plasma round to the face and still keep running. Not to mention that tyranid bio-engineering means they're filled with enough adrenalin and natural painkillers to melt a normal person.
And don't make me get started on the Ariadne effect, which allows them to survive Exterminatus.