Every size in a crochet pattern depends on one quiet assumption: how many stitches you make in 10 cm. Two people with the same yarn and hook can finish the same pattern at noticeably different sizes.
How to work a swatch
Chain 25 and work 15 rows of single crochet with the yarn and hook you plan to use. Lay the piece flat without stretching it, place a ruler across the middle, and count the stitches in 10 cm. Then count the rows in 10 cm.
What the number changes
If you get fewer stitches than the pattern lists, your stitches are larger, so the toy comes out bigger. More stitches means a smaller toy. Amigurumi also needs tight fabric, because gaps let stuffing show through. If you can see stuffing between stitches, drop a hook size even when the size looks right.
Using your number
Enter your two counts in the pattern tool and it recalculates every round to reach the height you asked for, so you do not need to change hooks to chase someone else's tension.