1) Maybe they're illegal conflict diamonds and therefore cheap since no one will touch them with a 10 foot pole
2) Maybe that 'diamond' is really a cubic zirconia and the girl doesn't know (Hey...it could happen - just because someone says it's a diamond, doesn't make it so)...the fakes these days are pretty good
3) Maybe they think that the bigger the stone, the longer the marriage will last
(sheeyah, right!)