Does how I hold the yarn really change my stitches?

Yarn feeds off the ball at a constant rate. What varies is how much of that yarn is already out of the ball and available to you. If your yarn hand sits 15 cm from the hook, you are pulling each loop against the friction of the ball and the loop comes up short. At 8 cm, the yarn is already paid out and the loop comes up to its natural height.

The same person, same hook, same yarn, can move a full hook size simply by changing that distance. Which is good news: it is far easier to change where your hand sits than to change how hard you pull.

Over the index finger: yarn over the top of the index finger, under the middle, ring and little finger — the most common hold and the easiest to keep an even feed. Under-and-around, with the yarn wrapped once around the little finger, gives more resistance and suits thread and fine cotton.

The pencil hold, with the hook held like a pen, gives finer control on very small stitches and is worth learning for amigurumi under 2 mm hooks. The knife hold gives longer strokes and suits tall stitches and fast work.

Work five sc, then deliberately move your yarn hand 5 cm closer to the hook and work five more without changing anything else. Measure the two groups. If the second group is measurably taller, your tension problem is positional and you have just found the fix.

Then fix it for good: rest the base of your yarn hand lightly on the work or on the table edge, so the distance stops changing as the fabric grows. Freestanding yarn hands drift further away as the piece gets heavier.