Generally, when a person bends to sit down, fat expands and muscles contract. This affects the hip and waist measurements and must be compensated for. Take a standing measurement and a sitting measurement, especially for large persons. Use the larger measurement for the actual measurement of the fabric and add seam allowances. You may allow a small amount of ease, but the body will only take up the amount of fabric that you measured at the largest sitting width.
One exception is for athletic wear. Measurements for athletic wear vary according to the sport.