It seems like armorstands with the "Small" tag's eye level is too far down. If you move an armorstand slowly into the ground, an armorstand with the "Small" tag will turn dark way before it's helmet (or block as a helmet) is under the ground, compared to the one without the "Small" tag.

Here is a short GIF to show what I mean:
https://gfycat.com/UnfinishedMaleArkshell

The small one turns dark before it's head is under the ground. The non-small one can have it's head go half way through the ground before it turns dark.

This makes creating "mini blocks" or similar difficult.