Well, I had an interesting problem recently. That might relate to this as it has to do with file size.
I built a detailed model of a portion of a ramp with a reinforced concrete slab on it.
I drew the reinforcing mesh with solid round sections (like reinforcing bar really is)
The total model had very few objects ... like 60.
I exported this to a 3d PDF and the file was hugely inflated and too big to send email with my isp provider though I had sent one the day before with 3100 objects.

I changed the reinforcing to square sections and had no problem.

In the first instance the converted pdf file was 20 times larger than the original 3dm file
After the change it was half the size of the original.

eric