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.