What's a not-too-huge piece of free (or very inexpensive) software that can deal with RAW files (so, not Gimp), and can do Lightroom-type tasks such as crop, rotate, watermark, spot and red eye fixes? Asking too much...
There are several good choices from the Mac App Store: Pixelmator ($29.99) and Affinity Photo ($49.99). Of course you have the new Photos if you're running Yosemite and while it's not as powerful as the two mentioned above, it does have good editing capabilities.
Also Darktable. Open source and free. I haven't tried it myself since I get the Adobe suite for free but it aims to replicate what Lightroom does. Affinity has been getting great reviews as well. I don't think it has the photo organization that Lightroom has though, it's more of a Photoshop replacement. Could be wrong though.