I sent an email to the guys from Buutvrij, and they gave me a choice: I could send it back and get my money back (which is not in the spirit of Buutvrij), or send the T-shirt back with a note saying 'Make my day' (which is in the Buutvrij spirit). So I chose the last one, and they promptly sent another package containing the longsleeve (which is larger in size [and more expensive]), along with the full array of Buutvrij-paraphernalia (stickers, booklets and a certificate).
The longsleeve was too small as well -- but it did make a good Sinterklaas present for Hans, my brother-in-law.
A few days after, I received an email from Buutvrij, asking whether I was satisfied or not, but due to various things I never got around to replying.
Today, there was another package from Buutvrij in the mailbox. It contained a new shirt, and this time, it fit! I'm totally amazed at the lengths these people go to, to keep their customers satisfied. And monday, I'll wear my new Buutvrij shirt to work!