>>> didn"t know this size/precision problem

It is not a direct problem with MoI, even Rhino4 will have problems correctly calculating a blend on those surfaces.

There are some other problems with construction on the model, as on the sweep, you are making a circle (top circle) sweep around a tight curve (at the top of the sweep) Care is needed as you can get kinks and therefore overlapping control points on the sweep surface, I know it can appear strange at first, but there are limits to every 3d application.

Up to now, you are doing very well.