The first winner has won 0.036 BTC with wagering 143 bitcoins and 1.43 expected loss. So I don't think 0.036 BTC is valuable amount. Also, the second winner has wagered 24.5 BTC and won only 0.019 BTC. If the prize pool is increased, I am sure more gamblers will be attracted.
The rankings are not in the control of primedice, they do not decide who will wager how much, of course it is great for them if people wager a lot but there is no requirement to wager that much to get the prize, people just bet way more than needed because they can and because they want to. So, what could primedice do about it? Tell people to not gamble that much and make them profits? There is no way a casino should do that, it would be suicidal, they are here to make money.