Don't worry, it's not you, it's Mapinfo.
The legend, particularly ones with pie charts, seem to be progressing backwards with newer versions of Mapinfo.
We've had real issues with the size of the pie just changing size both on the map and in the legend designer (but the 2 are never in sync) and then a completely different size on the layout window too!
You can try using the 'theme legend' instead of the 'legend designer' as the legend on your layout but even that is not always to scale.
The only other workaround we have found is to just create a completely separate map with pie charts and change the pie size until the new map's legend matches the scale of the pie charts in your actual map.
It's a shame that Mapinfo fails at something so basic as a legend on map.