I have always assumed that the restraint systems on rides are wildly over engineered to the point were, if it locks, its safe - regardless of weight. Last year at Alton Towers we went on Oblivion, and a plus sized lady was with us, the staff asked us to wait for the next train which has a plus sized seat and from what I could tell, it was the exact same seat as all the others with the exception that It had two seatbelts instead of one, but the purpose of the seatbelts, again as far as I know, is not to act as a restraint but to ensure the OTR is locked.
Basically the weight explanation on Terra Mitica's part sounds like BS...