A packer (100) for sealing in a tubular includes a mandrel (110) on which a packing element (150) is disposed between booster pistons (170). Each piston defines a sealed chamber (175) with housed areas of the mandrel. A setting force compresses packing element (150) between the booster pistons (170), which shear free. Ratchet mechanisms (200) including ratchet sleeves (210) and body lock rings (220) disposed between the pistons (170) and the housed areas initially allow the pistons to move in a direction away from the packing element (150), which expands the respective sealed chamber. Thereafter, in response to a pressure differential across the respective sealed chamber (175), the ratchet mechanisms (200) each permit urging of the respective piston (170) toward the packing element (150) while preventing retraction of respective piston (70) in the opposite direction.