I recently came across an image which showed a variety of Tefillin boxes that were much smaller than what I am use to seeing.
These looked to be pretty old so I took it as they might have been used by Jews forced to practice in secret and possibly took advantage of the size so they could be easily hidden.
I then came across another image of someone selling a pair of Tefillin that were smaller than what I have seen and owned. (Not my image so I blurred their face) They identify them as "traveling tefillin" and describe them as being for a traveling businessman or yeshiva student.
I've personally never heard of such a thing. Are Tefillin like this considered Kosher or is it something that shifted between different cultural communities (Say Ashkenazi vs Sephardic)?