I designed and printed a 5mm cube.
In cura everything is fine (5 x 5 x 5), but printed object is stretched in X (6.4 mm) and Y (6 mm) axis, Z is right.
I think belts are well tensioned, maybe too much, but surely not loose.
EDIT: After measuring different small objects it seems that they're scaled about 25% on both X and Y.
Sounds like the calibration factors for your printer need to be adjusted. I don't know your printer, but there should be somewhere in your setup that translates mm to stepper motor steps. If that is messed up, you will see parts which are scaled up (or down) consistently. If you can find the place to adjust the "steps per mm" for X and Y, then change the X number to one that is smaller by the factor 5.0/6.4, and change the Y number that is smaller by the factor 5.0/6.0 then re-run your test. Should get you close.