Project Management seems to leak potentially large amounts of memory

Let me demonstrate what I mean.
First I start up Qt Creator with an empty session. I have top running in a different terminal. The qtcreator process initially looks like this (only the relevant columns are included):

  PID      VIRT      RES      %CPU %MEM     TIME+    COMMAND
17435  744.4m 113.7m   0.0       0.7       0:00.61   qtcreator 

Now I start a reasonably large project, like Qt Creator itself (I'm switching to that project's session, to be precise):

  PID       VIRT        RES      %CPU %MEM     TIME+    COMMAND
17435  2759.4m 713.4m     0.0       4.5    0:18.28     qtcreator  

Then I switch back to an empty session. My expectation is that the memory allocated for the project is freed again. However:

  PID       VIRT        RES      %CPU %MEM     TIME+    COMMAND
17435  2759.4m 713.4m     0.0       4.5     0:18.98    qtcreator   

This looks problematic already. Suspicions are confirmed when switching to the other session again:

  PID        VIRT       RES      %CPU %MEM     TIME+    COMMAND
17435  3258.4m 1.093g      0.0      7.0       0:39.13   qtcreator 

This is actual resident memory. Repeat the procedure a number of times, and your system will start swapping and become unusable until the OOM killer comes to the rescue.

Additional observations:
1) The problem seems to be build-system independent. I originally noticed it with qbs projects. When I failed to find any leaks in qbs, I double-checked with qmake, and the same thing happened. Above numbers come from opening qtcreator.pro.
2) Some quick tests with commenting out some calls seem to indicate that neither the C/C++ nor the QML/JS code model are to blame.
3) valgrind does not report any memory leaks that could explain above numbers. I conclude from this that the resources are cleaned up on exit, just not during normal operations.