I think it's a psychological factor in play here. As you can see, we are still doing amazing, sure we had one very good jackpot (April 28 - May 4), but it was second round, surely a lot of people with no idea how it would turn out just jumped in with all they could risk.
I wasn't basing my view on the amounts, but rather the frequency of seeing new bubbles. I understand the price difference, but it seems like there's only a hand full of people getting any bubbles at all.
I would expect more random people to drop in an advertisement with just one bubble of the lowest amount.
A 5 BTC jackpot is plenty, especially since previous bubble games I've seen didn't even use a jackpot.
It would be nice if you'd reverse your display of the bubbles so the latest spend is on the first page at the top. I guess this would encourage more spends since they will have more views, rather than people coming to the site and seeing the first page or two as all the same thing. This wouldn't really effect the game play, and depending on how you're fetching the bubbles could be as simple as modifying your SQL query to 'ORDER BY id DESC' or whatever you use to select the current order. The only downside to that, is it may cause your 'top spender' ads to be less interesting. Then again, the people that spend more would still be in the top rotation after their bubble is pushed below what most people see when they first visit the site.