Here's my guess. People who give to Bernie believe in his message, and give a dollar here and a dollar there.
People who give to Hillary want something significant in return, should she take power. And so they give $5000 here and $5000 there.
We conservatives see right through Hillary. Now, here's the thing. The lefties are starting to see it too! That's why Bernie is drawing those numbers. Still, he's long shot. The Clinton machine is formidable.