Dear shelagh, The spot compression gives a better look at an area of concern on the mammogram. Sometimes the abnormality is due to folded over tissue, but this is not always the case, however findings from a mammogram can be more or less suspicious of a possible cancer problem. What the radiologist will be looking for is any changes, not only change in size. Again, depending on the findings of the repeat mammogram, whether the area is more or less suspicious will help to determine whether further evaluation would be advisable (such as a biopsy).
There is evidence that persons exposed to large amounts of therapeutic radiation (such as radiation for treatment of Hodgkin's disease - more common 10-20 years ago) may contribute to breast cancer risk. However, there is no conclusive evidence to suggest that exposure to "nuclear fallout" contributes specifically to breast cancer risk. There is a correlation between radiation exposure and thyroid cancer.
the thyroid is much more vulnerable to such exposures. As to compression views, it's hard to say without seeing the pictures. Often an insignificant density will become less noticeable with compression. Cancer may become more noticeable. If the density persists under compression, then what is done depends on how it looks: if there's concern, it can be biopsied under xray guidance. If it looks ok, and everyone is comfortable, then repeating over time is fine, and what you want to see is either stability or even regression.