The size of the file is caused by 3ds content unfortunately.
3ds file format is an ancient one and takes with it a lot of its problems - for example the 3ds format can handle triangles only - no quads, no polys.
Obj is as old, but little better - can handle polys or quads (I am not quite sure about the polys) - however the direct import of obj's was abandoned some time ago in AC unfortunatley, and due to the material definition sidecar file it is not handled correctly in some cases.
The current importers of 3d content base on pre 18 AC surfaces engine - so a lot of texture information cannot be imported.
3ds Max format is unfortunately possible to open by one application -Max -only due to licensing and proprietary of the format. No 3d app can open it.
And it is not that easy to export the content of such file because the creator of the Max dislikes any format, but the ones created by themselves
😜 especially they do not like the open formats
😜
The size of the file converted from any 3d format is sometimes huge because it is represented in GDL by the most primitive commands: VERT, EDGE, POLY, TEVE. The mesh is first defined by vertexes with their coords, then edges connect the vertexes, and then the edges create the polygons that cover the model, and all of this is pure text - so uncompressed can take a lot of space.
Save any quite complicated morph to GDL object and see for Yourself how big it can be.
Anyway the box created by box/brick command and the one created by primitives - say 10 time longer definition does not affect the speed of 3d generation - because even the higher level GDL commands are translated to primitives inside the engine, only affects the file size on disk.
The performace is affected in most cases by the no-good 3d content imported - like the holes in the 3d mesh and self folding mesh - a lot of free/ amateur made files are prepared without enough caution for such cases - as they do not affect performance of 3d modellers, but AC is affected very much in performance when having meshes with such errors.
I hope for better 3d format importers too, and ones with the 3d simplification/optimization - which is a big work to do.
I hope for correct texture defs especially when the object will be exploded to morph - now even UV map is lost
Best Regards,
Piotr