"Chiisai Oyaji Nikki" is a slice-of-life manga that centers around the daily life of a middle-aged man, referred to as "Oyaji," who mysteriously shrinks to a height of only a few centimeters. Despite his diminutive size, Oyaji continues to live with his wife and daughter, navigating the challenges and humorous situations that arise from his new perspective on the world. The narrative explores the mundane yet heartwarming interactions within the family, as well as Oyaji's attempts to adapt to his tiny stature while maintaining his role as a husband and father.

The story unfolds through a series of episodic vignettes, each highlighting the absurdity and charm of Oyaji's predicament. His small size often leads to comedic scenarios, such as being mistaken for a toy or struggling with everyday tasks like eating or bathing. However, the manga also delves into the emotional aspects of his condition, particularly the unwavering support and love from his family. Oyaji's wife and daughter play significant roles, showcasing their adaptability and care as they help him navigate his new reality.

Themes of resilience, family bonds, and finding joy in the ordinary are woven throughout the narrative. Oyaji's interactions with his family, as well as occasional encounters with the outside world, provide a mix of humor and poignancy. The manga balances lighthearted moments with subtle reflections on the challenges of aging and the importance of perspective, all while maintaining a grounded and relatable tone. The characters are portrayed with warmth and authenticity, making their experiences resonate with readers.