That seems awfully low, yes. But then the size shown in the export window is nothing more than a guesstimate. In the case of a film of mine it went from the near 6GB guesstimate to a mere 2.5GB, too, yes.
Do you maybe have an inordinate amount of black and/or talking head type material?
And you know that you can burn what I call a "mini BD" to a regular DVD using a DVD-R? Both in HD and playable (only) on a BD player. Only it's limited to about 30 mins. max. Maybe that's a viable alternative.