Honoo no Haramase Oppai Nyuu Doukyuusei

Kagami Kazuya has been living a relatively peaceful life at Shirosagi Academy, a school with an overwhelmingly female student body. One day, his father informs him of an emergency involving his mother. Before Kazuya can even process what’s happening, he’s whisked from his front door straight onto a plane, bound for an overseas destination.
Confused by the sudden turn of events, Kazuya finds himself on an international flight, enjoying a journey that takes him around the world one and a half times. During the flight, he watches a foreign film about a mad scientist’s “hyper-spatial car” that sends its passengers back to the Warring States period to help their parents—who were sworn enemies—fall in love and marry. As the movie ends, Kazuya drifts off to sleep…

When he wakes up, he’s back in the room he’d been living in just the day before.
Wow, modern airline service is really thoughtful, he jokes to himself, half-convinced it’s all a dream. To confirm whether he’s awake or not, he gropes the chest of a female dormmate and earns a slap in return. The pain confirms it: this is real.

But something’s off. The world he’s in now is subtly different from the one he left behind. Even his classmates have changed!
Now, his school is filled with busty classmates—each with measurements over 100cm. As his confusion deepens, the plot of the movie he watched on the plane flashes through his mind.
A multi-layered world… he concludes. I must’ve slipped into the space between them.

[Translated from dlsite]