User blog comment:Silva-zoldyck/what if... they caught annie before transforming?/@comment-7418318-20130924083233/@comment-7418318-20130925212904

Yes he transformed first inside the titan and later inside the (already weakened by Annie's stomping) tunnel.

But remember that both the police and the scouts locked him into a stable dungeon and the transformation experiments were to be done inside the tunnel shaft.

That's why I assume that if the surrounding structure is stable enough and/or undergound, the shifter will be unable to form a full body.