Photomatix side effect, minor resize occurring

[theregoesjb]

I am in the middle of PP for an image that i shot from a stationary point (camera was sitting on ledge), i used bracketed exposure and a 2 second timer to avoid camera motion.
I brought the 3 RAW files into Photomatix to create my HDR, I chose the "align source images by horizontal & vertical shifts". If I am to trust the absolute stationary-ness of my setup, should i not even bother choosing any "align" option?

Anyway, here is what im discovering:

To get maximum sharpness in one area of the picture, I am trying to bring in the Final HDR tiff into photoshop along with one of the original Raw file shots, I expected to be able to lay them directly on top of each other (hold shift-drag&drop) and then layer mask into the spots i want.
However upon bringing in the original, which fell above the HDR image, i noticed a slight border around it which turned out to be the edge of the HDR image beneath it. The HDR image somehow is *slightly* larger, by about 1/32" all around (9 pixels to be exact) which creates a total of 1/16" (or 18 pixels) stretch. This of course creates just enough of an offset that the images cant line up and i do not want to start masking through one to the other.

Why might this happen? could bit-depth effect this when i save the tiff (16bit vs 8) ?

 

[theregoesjb]

ok i just spend some time trying different combination of saving 8bit TIFF files and 16bit, as well as allowing photomatix to align the images and trying without any "align" option.
No matter what I do the photomatix generated HDR photo is ALWAYS larger in size (length & width). In one case it was by more than 1/2" larger and as stated before by as little as 1/16". Bit depth does seem to have some effect on the size, but kicking out an HDR TIFF the same bit depth as the RAW file does not make them the same size.