swell up, swelling, fat
臃 centers on abnormal enlargement: swelling up, becoming puffy, or accumulating excess fat. The idea is a body part that has expanded beyond its normal size.
臃 combines the flesh radical ⺼ with 雍, which likely contributes the sound. The character is associated with swelling or puffiness of the body.
The flesh radical ⺼ on the left suggests a body part, and the right side 雍 looks like a complex, puffed-up shape. Picture a body part that has swollen into a large, rounded mass.
For ヨウ, imagine a swollen body part that looks like a yo-yo: yo -> ヨウ, and the round, puffy shape reminds you of the swelling.