The Nokia Lumia 620 and the Nokia Lumia 520 are two of the more affordable handsets in Nokia's Windows Phone 8 lineup. The Lumia 620 offers the better display despite being smaller but that advantage results in it costing nearly 50% more than its sibling. However they both offer a near identical Windows experience leaving the user to select a device which suits their budget and personal preference.
You can read a detailed comparison between the Nokia Lumia 520 and the Nokia Lumia 620 on PhoneArena.