Question

When centric diatoms reproduce by simple cell division, the two new daughter cells are?

Answer 1

...are of unequal size.

Diatoms can reproduce either by sexual or asexual reproduction.
The asexual is the ordinary process and during the process of mitosis one of two new cells is smaller, while the other one is the same size of the parent cell.
The smaller cell will later  become normal sized because of the flexibility of the  walls or by sexual reproduction.