Stack inserter with small stacks

[Chindraba]

The stack inserter, at full capacity, can hold 16 items, and will do so if the belt stack size is 2 or 4 (and only moving 15 if the belt stack size is 3). There are, however, items with a stack size of 1, such as nuclear fuel. 40 fills a cargo wagon and 48 fills an iron chest. While I've never tested it, I've always thought that the other inserters only move 1 item at a time, or for bulk inserters with a capacity of 12 they only moved 5 or 10 at a time when the item had such a stack size.
Does the stack inserter still move 16 at a time, and make stacks of 4 on belts when the item has a stack size of 1? Or, is the item's stack size a limit to the stacks on the belt and the number of items the stack inserter can move?

[tulpabug]

I believe 1 stack items have an absolute max of 1. But maybe they changed the rules for the new stack inserters.

[Simon]

If an item has a stack size of 1, a stack inserter will only grab one, and thereby cannot stack anymore than one, so you can't stack rocks in space, because they have a stack size of one.