Were all the shirts the same sizes? Did they provide the design or did you create it yourself? How were they customized? Heat transfers, vinyl, screen printing, embroidery - you did not say.
If I were screen printing them, I would size it to look "good" on the smallest size shirt. I have done jobs that ran from youth small to adult 4xl, they all had the same design and design size. On the larger sizes, it will be smaller as there is more real estate. I will point this out to the customer and if they want (and will pay for it) I will make a second size for the adult shirts. Remember too, there is a 10" difference in width between an adult small (18" chest) and a 3xl (28"chest). That is a lot of real estate variance. A design that "fills" the small shirt will look small on the 3xl.
That is why I always design to fit the smallest size garment. I show the customer how it will look, and then let customer choose if the want one or more design sizes - AND what that choice will cost. Most times, they will pick the same size design for all as they don't want the added expense of the second design, especially for a small run.