I think what he's saying is: Given a balance of $50, they "earn" $5/mo. Given a balance of $200, they "earn" $10/mo (or $199*0.10 + $1*0.05). If they don't spend it, I'm assuming it's "rollover", and if they eat into their capital (eg: buy an xbox) then they feel the sting of earning less-per-month.