Depends on the heterogeneity of your phage population. Also could be an artifact of the method where phages are diffusing out of the agar layer at different times.

Plaque means pure colony irrespective their size because the size developed ac ording to age and growth rate.

Varied plaque size is normal with many phages, even with a pure stock. You could for example pick one small plaque and one large plaque and plate them out, my guess is you will see the same variation in size from both.