From this we can see that diatoms reduce their size after asexual reproduction.
This would ultimately lead to nonviable small size. Is there any mechanism to avoid this ?
The ability to reproduce sexually is closely associated with cell size of the diatom. It is the most prevalent method of restoring cell size, and it is often triggered when the size of a cell is less than a critical level (usually smaller than half of the original size for most species).
Source : Sexual reproduction in diatoms