Here's a little something I just discovered. The "bounding box" traces, in all views, pretty much the outline of the default datums for a part and the diagonal cuts through the csys. In this ancient, crude and undeveloped functionality, when the part grows (including when it grows asymmetrically, the datums grow with the features, but they grow SYMMETRICALLY to some distance outside the furthest limit of the part and still passing through the csys.
Do this little experiment to see, graphically and in the extreme, what I'm talking about. Start a part in the usual manner. Make some simple geometry (circle, rectangle) OFFSET from the default datums by about the width of the feature. Now, do 'Info>Model Size' on the resulting solid. The "bounding" box will encompass offset datums, csys and the furthest limit of the part. In other words, it will make a box of the default datums (three sides) and the part (three other sides).
This functionality pretends to measure the size of your part but it just measures the size of the datums extended the length of your part, the diagonal passing from or through the csys. Don't know really what good it is to know the diagonal since there are an infinite number of boxes that could make the same one. Seems like more an artifact of the early ooh-ahh factor of modelling. So, while it might not be much, the claim constantly held: "you can't do THIS in 2D". And Pro/e\PTC have lived on their minimal investment in the functionality, profitably, for 15-20 years.