Aperture = Focal Length / Diameter. That's the simplified formula.
Lens designers of macro lenses have two basic choices: they can (1) extend the focus barrel to get close focus, which would tend to double the lens length at 1:1; or (2) they can shift the focal length slightly to keep from extending the lens much (if any). Most macro lenses use a bit of both strategies. But if you change the focal length without changing the diameter of the lens opening (that would be a real trick), you're going to change the maximum aperture rating of the lens. In real life, the lens makers do a bit of both, but that means that as you focus closer you will find the maximum aperture you can set changes. Here are some of the differences for the Micro-Nikkor lenses:
- 40mm: infinity f/2.8, closest focus f/4.2
- 60mm: infinity f/2.8, closest focus f/4.8
- 85mm DX: infinity f/3.5, closest focus f/5
- 105mm: infinity f/2.8, closest focus f/4.8
- 200mm: infinity f/4, closest focus f/5.3