narrow, small
褊 centers on restricted physical extent: something narrow or small in size. The two given meanings are direct expressions of this same idea of limited space.
褊 combines 衤 (clothing) with 扁, which likely contributes the sound and carries a sense of flatness or narrowness. The character originally referred to narrow or tight clothing, later extending to narrowness in general.
The clothing radical 衤 beside 扁 suggests a garment that is too tight or narrow. Picture a shirt that is too small, squeezing you uncomfortably.
For ヘン, imagine a narrow henhouse where the chickens are squeezed together: hen -> ヘン, and the cramped space matches the narrowness of 褊.