The title sums it up.
I have some polygons. I did an > Add field >> Calculate area
I edited them and a few became significantly larger.
I added another field, calculated the area ... and I get the same results.
?
Maybe you calculated the coordinates in the units of the data frame or the other way around... you can have unprojected data in a projected frame and vice versa, so you will have to figure what is what
perhaps a few screendumps of the used coordinate systems will help.
Another suggestion: store your data in a (file) geodatabase instead of using shapefiles. In a geodatabase the area is auto-calculated
