If you have stirred for about the same time as I do, there are a few reasons for the size.
1. Stirred too hard for too long. This shatters the curd into small units and you loss a lot of the captured fat. During the stirring phase your whey is probably looking milky instead of a clearish liquid with a yellowish/greenish tinge.
2. Temperature control might have been a little fast.
3. If using P&H (Pasteurised and Homogenised) milk, then might have been over processed or the amount of Calcium Chloride added might need to be adjusted.
Best rule for stirring is to only stir as gently as necessary to prevent the curds from clumping. Curds a very fragile during the first part of the heating and need to be carefully looked after.
I tend to just insert the spoon to the bottom of the pot and lift to the top and repeat more than a circular stirring.
Hope that helps
Mal