Running

/summon Slime

will summon a slime of random size, but saying

/summon Slime ~ ~ ~ {}

will always summon a tiny slime. A tiny slime will also always be summoned if there are tags in the curly brackets that don't specifically specify a different size. I don't think this is intentional, and if it is it would be really annoying.

Steps to reproduce:
1. Place a repeating command block with

/summon Slime ~ ~1 ~

2. Power it.
3. Notice how the slimes are of random size.
4. Unpower it.
5. Kill all slimes.
6. Add {} to the end of the command.
7. Power it.
8. Notice how all the slimes are tiny.
9. Unpower the command block.