You can certainly get MRI size assessment changes for a variety of reasons:

1) The slice location of the image (is it along the greatest diameter or just to the side of it)

2) Measurement criteria (do you include the IAC portion or not)

3) Measurement point (do you go to the middle of the tumor outline, just inside or just outside when performing a measurement)

4) Do you measure perpendicular to the axis of the IAC or the greatest diameter.

Fortunately, all these can be accounted for. For the first, taking the measurement for the slice either side of the one that has the greatest size will tell you if your at the greatest diameter (the slices either side will be roughly equal) or you'er off to the side (the slices either side will be quite different). There are methods that take the greatest diameter slice and the diameter of the slice either side that work out what the real greatest diameter is. Don't ask a radiologist how, it's more a job for a mathematician (or a regular person who can do a curve fit in Excel).

For the next three, give the older and current images to a single radiologist to measure again. This way you don't get inter-operator variation.

Or, just get the CD and take a look yourself. Does it look bigger?