Why does my chilli plant have small leaves?

Small, puckered, or distorted chilli leaves are most commonly caused by broad mites or thrips — pests so tiny they are invisible without magnification. Spray neem oil + soap solution (5 ml neem + 2 ml soap per litre) every 5 days for 3 cycles. Other causes: magnesium deficiency (spray Epsom salt solution — 1 tsp per litre) or extreme root-bound conditions — repot into a larger container.