The word **膨胀 (péngzhàng)** is a compound formed by combining two characters with closely related meanings to express the concept of expansion. The first character, **膨 (péng)**, primarily conveys the idea of something becoming physically larger or swollen in size. The second character, **胀 (zhàng)**, emphasizes the internal cause or state of that expansion, often related to pressure, fullness, or distension (like a balloon filling with air). Together, they create a synergistic term that means "to expand," "to swell," or "to inflate," describing both the outward physical increase in volume and the internal pressure that drives it, and it is commonly extended to metaphorical contexts like "inflated ego" or "economic expansion."