User blog comment:Nobody700/What If: Alien Invasion arc part 2/@comment-5038468-20151219213055/@comment-162.199.72.31-20151219231445

He didn't have any reason to expand the gravity field beyond that room, he was trying to kill Saitama not sink his ship.The fact that he's ranked as dragon means there's a high probability that he can use the field on wide scales like that.