I noticed that one edge (which is also the starting point of the print) is always bigger than the other three. Additionally, the walls do not have the same thickness as well. The wall thickness (starting from the thick edge) starts very thin and gets thicker till reaching the thick edge again. Does anyone have a clue whats the issue?
As Fernando suggests, the problem is likely with your source file. One easy way to verify this is to slice twice, rotating the STL by 90 degrees. If both prints produce fat/thin sides in the same dimension on the print bed, then it's a printer problem. If the fat side rotated with the STL orientation, then the STL is at fault.