Thank you for answering so quickly!
I have changed the yarn, but not the hook, so I crocheted it with the same hook (I have just this one hook for the moment), but still there is a difference in size.
But normally, the second square should be SMALLER than the first, as it is meant to be crocheted on smaller than G needles.
Maybe it is just that I crocheted the second square less tightly, which I do not think I did.
As to blocking, I've read that one can't block synthetic wool, so that's not an option.
Your ideas about turning it into some decorative effect are very lovely! Thank you for the ideas!
I noticed that it just LOOKED bigger at the beginning and that, after the last round, it was about equally big as the other square.
Thanks anyway for your help!!
(Fixed mistake: I used "knit" instead of "crochet")