achaparrarse

To become stunted, squat, or stocky in shape — used for plants that grow low and bushy instead of tall, and informally for a person becoming short and thickset, from chaparro (short oak shrub, or a short/stocky person).

• El árbol se achaparró por el viento constante. — The tree grew stunted and squat because of the constant wind.
• Con los años se fue achaparrando un poco. — Over the years he got a bit stockier and shorter.
• La vegetación se achaparra en zonas de mucha altitud. — Vegetation grows stunted and low at high altitudes.

It describes a stocky/squat shape, not weight gain in general — don't use it as a synonym for engordar (to put on weight).