Quite often, plants with big fat leaves will change to small narrow leaves during flowering.
All this is natural and it will not affect your plant negatively.
From my experience, I think this occurs because the chances of getting pollinated with thin leaves is probably higher than with wide, fat leaves.
Some Indicas, with wide thick and large leaves, often end up looking like Sativas during flowering, with long, thin and smaller leaves!
Not to worry, your plant "knows" what to do.