They're made through Teespring, a kickstarter-type site for T-shirts. They handle everything except design, so printing, delivery, website, etc. If Wink/John/Caleb were to have gone through a traditional screenprinter (which would have cost them money up front, instead of having us pay for them), they could do whatever they wanted. But, this way is more convenient, more cost effective for them, and has someone else handling the busy work.
Contact Teespring and request that they carry larger shirts.