The dollar values may show this, and these numbers show you're right. However, influence wise, unions have massive sway because of the breath / consistency of their voter block - which provides a higher ROI on their lobbying money.
13 out of the biggest 20 donors are unions. http://www.opensecrets.org/orgs/list.php?order=A
Donations are only one small way that money influences the hill. If they were as effective as lobbying, you wouldn't see 10X the money being spent on that.