Are those photos with the extension tubes cropped/blown up at all?

Yes, some of them are cropped a little. The limitation isn't the magnification, but actually how close you can get to the animal and still get lighting on it as well. Often if you get too close, the lens will shade the lighting.

If I'm to understand, extension tubes allow you to focus closer, correct? As in, if the normal closest distance to focus is 1 foot, by adding extension tubes, it'll allow you to go to, say, 6 inches?

I don't know exactly how much closer (it depends on the lens and amount of extension), but yes, you can get closer and therefore have higher magnification.
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Chris Harrison
San Antonio, Texas