What is the reason for 'microcytic' anaemia?
The reduced iron availability affects the red cell size during differentiation.


  The presence of red cells with reduced mean corpuscular volume (MCV) reflects reduced hemoglobin synthesis, which can have several causes. It can result from defects either in iron acquisition or availability, or in heme or globin synthesis. Experimental data in mouse demonstrated that early mitotic divisions are accompanied with no reduction of MCV, whereas during differentiation mitotic events are associated with a substantial reduction of MCV[1]


http://www.ncbi.nlm.nih.gov/pmc/articles/PMC2649346/


Coopersmith A, Ingram M. Red cell volumes and erythropoiesis. II. Age: density: volume relationships of macrocytes. Am J Physiol. 1969;216:473–82.