Hydrangeas

Hi  took 6 cuttings last year one was a rooted cutting every one took, the rooted cutting and one other is in a border  and the other 4 are in pots all doing well what I can not understand is why some in pots have very large flowers and the others are normal why the difference the plants I took the cuttings of are only normal plants and they are in large pot which I have had for 8 yrs  and their flows are not large and never have been