Hi sunspeed, the size shown on the top level of the properties panel is an estimated size done with a quicker calculation. For solids and surfaces it is the bounds of the display mesh and for a curve it will be the bounds of points sampled along the curve.

Because that piece of UI is updated on every selection action, it needs to be a quick calculation. Otherwise there could be a kind of laggy or hiccupy feel with more complex models where it would be doing a lot of work on every selection click.

To get a higher accuracy size you can click the size label and the "edit size" menu will pop up. The values on the edit size menu are done using the more complex full analytic calculations on the spline surfaces or curves.

- Michael