Why does my cactus go skinnier at the top than at the bottom?

Question

It gets even sunlight and i water it as appropriate.

Answer 1

Is that some kind of metaphor?

Answer 2

Biomechanics... imagine a tree that was skinnier at the bottom than at the top -- how sturdy would that structure be?

See: http://www.amazon.ca/Plant-Biomechanics-...

With time (and further growth), you will probably see that same area (height from bottom) "fill out" and the newer growth near the top will be more slender than it, and so on.