Jesus' criticism that some religious leaders make their phylacteries broad concerns a recognizable Jewish devotional practice. Phylacteries were not invented as an object of hypocrisy; they expressed a serious effort to embody commands from the Torah.
The criticism falls on enlarging visible religious signs in order to be noticed by others.
Passages in Exodus and Deuteronomy command Israel to bind God's words as a sign on the hand and as frontlets between the eyes. Jewish practice came to embody these commands in small leather containers, later commonly called tefillin, holding passages of Scripture and worn during prayer.
Jesus' reference assumes the practice rather than condemning it. The surrounding criticism concerns doing religious works 'to be seen of men,' so the exaggerated size of a devotional marker becomes an example of public display.
Understanding the practice prevents the verse from sounding like ridicule of an unfamiliar Jewish object. The underlying act was rooted in Scripture and in remembrance of God's commands.
Jesus' warning is consequently more searching: even a legitimate religious practice can become a means of seeking status when its visibility matters more than the obedience it is meant to represent.