Frederik
I love how you say "you didn't do anything other than Export, Import and Export it again" ......
I have not done this personally, however I am pretty sure that I know what "you did actually do here ..."
When you started out with a set of contours - you have to look at what type of object they were in TBC. The different types of CAD Objects Polyline, 3D Polyline and 3D Line have different amounts of data associated with them as do Polyline, 3D CAD Line and Linestring in TBC. For example a Polyline can be a series of 2D Nodes and either No Elevation or One Elevation that applies over its full length - this makes it a relatively light object in terms of data size. If you now make that a 3D Line, it now has a series of XYZ nodes where Z is stored at every node - those are all extra bytes that are stored with the line. Depending on whether you work at sea level (0.000) or in the Mile High City of Denver 5280.00' there is a significant size difference in the number of Z Bytes in the file. The more nodes you have the more Z Bytes you have. A CAD 3D Line also is stored as From node 1 to Node 2 and Node 2 to Node 3 and Node 3 to Node 4 so every node except the first and last node of the lines is actually stored twice which ~doubles the size of the data in the file, and if you also factor in all of the Z values that increases it further again.
So then you have to look at what TBC does when it sends a selection of data out via a DXF or DWG file. In TBC v5, there is something that changed and that makes the files all bigger because we are for some reason adding a bunch of Civil 3D data out with every data selection - I did a simple test the other day on that and a line from BC-HCE was a few KB and the same line from TBC v5 was a few MB in size. However neglecting that something that changed, what is happening here is that you take a 3D line in TBC and it has a fixed Z on all nodes, and output it to a DWG - it becomes e.g. a 3D line in the DXF. When we read it back in again, we recognize that it is actually a 2D line with fixed height so we make it a Polyline. Then when we Export it again we now export it as a Polyline which makes it smaller and that is why you are likely seeing the difference in size.
The file size change is for sure caused by a difference in the data. The challenge in TBC is while we can convert data from "basic data types" like a Polyline to a "sophisticated / richer data type" like a Linestring, we dont have any way to downgrade data in the reverse direction, however the CAD Export / Import / Export process that you tried does do that (because CAD data is significantly dumber than TBC data.
While I have not tried this myself - I am pretty sure this is what you are seeing
Alan