The size of the exported JT files is roughly 10 to 15 times to big. It look like that the building data use a configurartion which may fit in engine manufacturing or rocket sience where fine granularty and exact geometriey is requiered. At least the number of created elements in building and architectural data have to be reduced to get JTs which are perfomant in viewer application.

(Test environment: Microstation Connect v17)