Hi,
Do you see that we can have DRC error when it is a square compare to if we draw a circule (e.g. a square cover more area than the circle).
The GDS might not be exactly the same as mask, but not in the way we discuss here. OPC might be performed. Also, there is also some sizing for different layer (e.g. it is difficult to draw 0.101 poly, but easy to draw 0.1 poly). Lastly, the mask might be 4x larger, but this does not affect the final chip.
Regards,
Eng Han