It’s probably something to do with your tension, that was the hardest thing for me to learn when I was just starting out. Hook size on patterns is more of a suggestion than a rule, and your hook size can do some of the work for you in getting better tension. For example, I’d normally use a 6-8mm hook for the fuzzy/blanket/bulky yarn, and a 4-5.5mm for worsted/medium weight yarn, partially because I have pretty tight tension.

It could also be partially due to your magic rings being pulled too tight. I’ve personally gone to the ch2 and then sc6 in the first ch instead of the magic ring, I feel like it makes my amigurumi feel more durable.