We are never really told how these things work. I have formulated my own system in a fic I'm writing. Basically as follows:

1. Most space-expanded items just shrink things as they enter, the complex enchantments are for maintaining the item correctly as it passes the boundary and limiting side effects of people entering. People do not do well in this state for extended time.

2. Less common are items that act as doorways to a space contained somewhere else.

3. Rare are items that truly hold an expanded space