Sabikui Bisco, Vol. 5 (light novel) — Shinji Cobkubo

WE'RE GONNA NEED A BIGGER BOW…
Hot on the trail of their friend-turned-foe, Shishi, the new leader of the Benibishi, Bisco and Milo find themselves all the way in northern Japan. But how are they supposed to stop the blossoming berserker when Bisco is the size of a toddler?! The ravenous blossoms plaguing his body have severely diminished his strength (and size), and the pair will have to track down a mysterious mushroom in the Hokkaido region to cure his ailment. There's just one problem with that. Hokkaido is…alive, and it's very, very hungry.