Joseph………
I like to have a set at home, a set I keep in shul and a 3rd set for travel (I used to keep these in my Florida home, until I sold the home).
The straps on my Bar Mitzvah tefillin are not the length I require to be comfortable around the girth of my adult arms (quite bigger in circumference than they were at 13).
The batim that were in fashion 50+ years ago were much smaller than those typical today, and I have bought newer ones that are more in keeping with my general size.
Similarly, I now wear a bigger talis than the one that was purchased for use at my Bar Mitzvah (purchased by the German side grandfather).