as in, 'to drop a...'
A desperately needed and massive dump, sufficient to reduce one's waist measurement by 1-2 inches (2.5-5 cm) and/or relieve the pressure of a tight waistband after a good meal.
See also man diet.
(From the Kellog's 'Drop a jeans size' dietary advertising slogan)
Oh, he's in the bog, dropping a jeans size.