Question

As amount of grain boundary area decreases with increasing grain size

Answer 1

True, but what grain boundary are we talking about? In ferritic steels, you have ferrite grain boundaries and prior austenite grain boundaries. It has been said that the best feature to stop a propagating cleavage crack is a prior austenite grain boundary. But you can increase the grain boundary surface area without refining (reducing) the size (volume) of the austenite grain. The ferrite grain effect is very dependant of the final microstructure - polygonal ferrite, or one of the many variations of lower temperature transformation products.