I just spent the morning trying to determine why an assembly file size was larger than one gigabyte, when the assembly consisted of about 30 (primarily prismatic) parts with a few downloaded McMaster Carr models. One configuration, no external references, really simple. The culprit turned out to be this guy:
The model itself is about 10MB, but if I drop it in an assembly and save, the assembly file size is 900MB. The assembly I was troubleshooting had this cord grip in a subassembly, but strangely, the subassembly file size was OK. Only the top level had the problem or if I drop it into a brand new assembly.
The offending feature was (not surprisingly) the braid. It is modeled as a single helix which is then feature patterned and mirrored. Switching to a body pattern and mirror solved the problem.
If you have some free drive space and some time, give it a try.
This was using SW2017 SP5.
I'm now going through our PDM vault looking for other unusually large assemblies. We have 50+ that are 400MB or larger, which is suspicious since we don't work with complex models or enormous part counts.