Advantage: E-P1.
Refer to DPReview’s comparison.
Weight (inc. batteries)
E-620: 515 g (18.2 oz)
E-P1: 355 g (12.5 oz)
Dimensions
E-620: 130 x 94 x 60 mm (5.1 x 3.7 x 2.4 in)
E-P1: 121 x 70 x 36 mm (4.8 x 2.8 x 1.4 in)
Simply put, the E-P1 is smaller and lighter than the E-620.
Read more on Olympus E-P1 vs Olympus E-620.