iPhoto export shrinks images (and shows inconsistent file properties)
"The path it's taking me to in Finder is obviously not the original." It is taking you to a location that looks like this? 
Also, Depending on how your exporting may change the characteristics of the file format, hence changing from the "Original" and the original location. 
It should also be noted that PhotoStream can change the size of files. For example, if you take a photo with you iPhone and sync via cable to you Mac, but then alter the photo in an iPad using iPhoto, what you get is a different sized file. There's a similar discussion here.