How do I reduce size of imported models

Hi John,
indeed, it is possible to save an entire assembly as "one part" and also specify to save only the outer components or only the outer surfaces.
The "size reduction" with respect to the original files is variable and depends, precisely, on "how much interior geometry" we have saved.
I will give you an example, not very representative, of an assembly for a small mechanism that has almost no internal geometries, but that, in any case, shows a reduction in size when saved as a part, keeping only external surfaces.
Hope this can help you,
Kind regards!

If the above doesn't give you the size reduction you need, you can adopt a "compromise" that I particularly use a lot: create simple replacement geometries and apply images of the original complex geometry to their faces.

I use this option when I don't care about the details of those components as they appear in an assembly where they are not seen at a small scale, so as to perceive that the true geometries have been replaced by images.

In your case, it would be something like creating a box and drilling holes in it (to connect the pipes) and then applying the images you get from the real views of those objects to its 6 faces.

In short, it is not wonderful, it is not beautiful, but when my machine does not achieve "the wonderful and beautiful" then I settle for "the possible and practical".

In these images a set is seen in its complete, original version, and this is how it is used for its mechanical description. However, when the same set is used for a "macro" logistic description, the number of these sets is very high and it became prohibitively expensive for me to use their full versions (which I then replaced by a simple externally mapped geometry with images of the actual geometry).