That's how JPG works. The compression is redone every time the file is saved. When you export, you create a new file. If you create a JPG, it will be created to the compressing setting you use, but that compression setting is specific to the program using it, and will not result in a file of the same size saved by a different program.
The only way to get the exact same file is to export the file you imported -- in Aperture, this is done as "File➞Export➞Original".
If you are aiming for the highest quality JPG that fits within a upper limit of storage required (e.g. 5 MB), you can make small changes using "Size to" set to "Percent of Original" and then trying, for example, 90%. For almost all uses a quality of 11 is more than sufficient. For printing (where max IQ is primary, and file size shouldn't matter) use TIFF 16-bit.