You press and hold Alt, and click on a photograph.
You may think that part of the photograph is sampled.
That's because of the term, sample.
And, perhaps, because the sampling cursor is circular.
Let's say we sample the above forehead.
It's easy to think that the sample looks like the image below.
And, it's easy to imagine this sample getting bigger or smaller depending on the brush size.
As you probably surmised, the above is incorrect.
A sample isn't a sample.
A sample is a center point.
When you sample, you're choosing a new center point for the entire photograph.
When you enlarge the brush, you get more of the photograph, from the new center point.
For example, below, a point between the eyebrows was sampled.
On the right side, above, the sampled area was cloned using three different sized brushes.
As the brush size size increased, more of the photograph was included.
With a small brush, you get a small portion of the photograph.
With a large brush, you'll probably get to much of the photograph.