@vadim , Thank you for the reply, i have got some idea .

Sintering brings more of physical changes while calcination brings physico chemical changes in the resulting solid state products.

Also at higher calcination temperatures, smaller crystallites agglomerate to give grains of bigger size. Grain size is more conversent with material / mineroligical definitions of crystal grwoth while crystallitesize is the frequently used term in basic scientific definitions more commonly in nano structures.