dilate, distend, bulge, fill out, swell
脹 centers on expansion from within: swelling, bulging, or distending, whether a body part, an object, or a feeling of fullness.
脹 combines ⺼ (flesh/body part) with 長, which likely contributes both sound and the idea of lengthening or expanding, giving the sense of bodily swelling.
The flesh radical ⺼ beside 長 (long) suggests a body part growing longer or larger: swelling or bulging out.
For チョウ, imagine a long swelling named Chou: Chou -> チョウ, and the flesh part expands like a balloon.