Effect of forced breathing on RBC size
Forced breathing, a.k.a hyperventilation, causes hypertonic dehydration.

According to Greger and Windhorst, Comprehensive Human Physiology (2014):


  Loss of more water than NaCl leads to hypertonic dehydration and hypernatremia. In this condition the intracellular volume is marked decreased, whereas the extracellular volume is less compromised. Possible causes of hypertonic dehydration include excessive loss of hypotonic fluid (e.g., sweat) or hyperventilation.


Therefore, the decrease of the size of red blood cells is due to the dehydration following hyperventilation. 

Source:


Greger, R. and Windhorst, U. (2014). Comprehensive Human Physiology. Berlin: Springer Berlin.