The problem is that the 3DS format itself is unitless. It has no way to specify a real size. There is no formal specification or standard for 3DS, and so there is no accepted scale factor at all. So of course, every application that uses 3DS does it their own way. The majority of CAD and 3D modelling software allows the user to choose whatever units they deem most appropriate when exporting to unitless formats like 3DS. Weirdly the Vectorworks 3DS exporter doesn't do this. VW has arbitrarily decided on mm, so you have to manually work out the scale factors if you need a 3DS export to be in metres or inches. Sadly this isn't documented or mentioned in the 3DS exporter dialog. The GDTF Builder has arbitrarily decided that all 3DS files are drawn at a scale of 1 unit == 1 mm, presumably because Vectorworks happens to do that. This is of course often going to be wrong! Applications consuming unitless formats (like 3DS) require a UI for the user can specify which units they exported in. This is another example of why the 3DS format is not suitable for use in GDTF or MVR, and should be deprecated ASAP.