I don't think we have a loose belt here....particularly as the photo clearly shows an over-extrusion on the corner of the layer where the extrusion starts and finishes.
More investigation is needed now to establish if the issue is either at the start of the layer....or at the end of the layer, and I think some reading is needed to help. Have a careful read through the first post in this thread: viewtopic.php?f=8&t=1980
Start from the heading "Ooze control".
What I believe is happening is either the extruder is still extruding when the head comes to a stop after finishing the perimeter around the cube....or during the move from the end of one layer to the start of the next layer (which may simply be lifting up a layer thickness) there is still some filament being extruded, or if there is any retraction (ie withdrawal of the filament before the nozzle moves, and return of the filament after the move) then a blob of molten plastic is being extruded. Either of these situations will result in the extra plastic resulting in the oversize you are describing. If you're lucky, you might even be able to see which of these is happening by watching the print in action. If not, you need to experiment.
If you've understood the description of the process and what bit each setting affects, you will see that you can adjust or introduce coasting to alter/remove any former over-extrusion, and adjust or introduce retraction and/o extra restart to address any latter over-extrusion. You may find that retraction is already in use as your printer utilises a bowden cable , so adjustment of the existing values may be needed.
I'm afraid it'll be a tweak and see exercise.....but remember one rule. Never adjust two settings at the same time. Why? Because you will not know which one actually caused a change! Be patient, and you'll get there with a bit of luck.....and report back your findings.