Photoshop for All: [Adobe_Photoshop_Techniques] JPEG size and resolution vs file size

Hopefully someone in the group can clear up some confusion for me.
I am using CS5 but I think this apples to most versions of PS and PSE.
I set my image size and resolution and crop, example:
w-4"
h-6"
300ppi.
When I save as a JPEG, I an give a dialog box asking to select file size from small to maximum. The example will yield the following results for file size:
Large - 1.45 MB
Medium - 157 KB
Small - 85 KB
When re-opened in PS, all will still show as having the original settings of 4" x 6" x 300ppi and when zoomed to 100%, seem to have equal detail.
How can different file sizes contain equal amounts of information?
What am I missing here?
Thanks,
Bruce A.