The mean corpuscular volume is the most important of the red cell indices. It measures the average size of the red cells.
A red cell is mainly made of hemoglobin with a membrane to go round it. Small round cells are caused by a lack of hemoglobin. There are only three causes of small red cells - there are two causes of too little hem and one cause of too little globin. The amount of hem is controlled by iron and the amount of globin is controlled by the genes you inherit from your parents. If you inherit dodgy globin genes you have thalassemia. There are several sorts, but they all result in small red cells.
If you have too little iron you can't make hem and you end up with small red cells. Too little iron may be the result of too little iron in the body - either because of not enough iron in the diet, which is very rare in most Western countries, or because of excessive iron loss - mainly due to bleeding. But too little iron for the red cells can be caused by the macrophages greedily hanging on to it and not releasing it to the forming red cells in the bone marrow. This happens with a wide variety of chronic disorders such as Crohn' disease, ulcerative colitis, rheumatoid arthritis, chronic infection and some cancers.
Raised MCVs have more causes. Everybody knows about megaloblastic anemia due to a deficiency of either folic acid or vitamin B12. These are relatively easy to diagnosed and it is important to rule them out by measuring B12 and folate levels, but in actual fact, these are relatively rare cause of macrocytosis, as too are the floppy membrane diseases - conditions where the size of the red cells is increased because there is too much membrane. Examples would be liver disease and thyroid deficiency. Alcoholics have large red cells, probably for a variety of reasons, and probably the commonest cause in older people is MDS.
However, we sometimes see raised MCVs in CLL and none of these reasons apply. I think that whatever fills the bone marrow spaces that shouldn't be there causes an increase in the MCV. You certainly see it in myeloma, but why this should be is unknown.