But Hillary Clinton still raised more money in both quarters. So what good does Sanders's small donor army actually do?
Instead of thinking of the total amount raised, consider the number of donors and how much the campaigns could raise from them. Below are pictured all of the donors to the two candidates who gave at or below the maximum in the second quarter. (The horizontal axes reflect the percentage of each campaign's donors who gave up to a certain amount; the campaigns didn't have the same number of donors.) The light blue area is how much more could be raised if all of the donors gave the maximum. The dark blue is what the campaigns have already received.
(Notice that you can see little shelves at every $500, since people like to give in even numbers.)
That's just the donors that were included in the candidates' quarterly filings, but the point should be obvious. The amount of area in light blue for Sanders is five times the area in dark blue. For Clinton, the area in dark blue is larger -- meaning less money to wring out of people who've already ponied up once.
It's certainly not the case that everyone -- or perhaps even a significant portion of Sanders's supporters -- can give $2,700 to a political campaign, of course, nor is it the case that the candidates can't expand their pools of donors in successive quarters. What the chart shows is why it's likely easier for Sanders to add onto his fundraising totals by hitting up the same donors than it would be for Clinton.
Which -- per reported third-quarter fundraising totals showing Sanders raising nearly as much as Clinton -- appears as though it might have been the case.