Nokia Lumia 520 have 37.78% smaller semiconductor size/newer CPU than 620, also 2.05% less body volume. Lumia 520 also 3 g lighter and 1.1 mm thinner than Lumia 620. But 620 have 246 ppi when 520 have 233 ppi of higher pixel density (5.58% higher).
Noia 620 has a flash is good for low-light environment and can be often used as flashlight, a front camera can be used for video chats, also has Near-field-communication (NFC) allows wireless transactions like payments.
So, I think Nokia Lumia 620 is still better than Lumia 520. Also 620 has a front camera can be used for video chats. Its more fun.
Nokia Lumia 520 vs Nokia Lumia 620