If the sides of a calibration cube are a little bowed, there are a couple of things that I would look at.

• e-steps/rotation distance (Klipper): if your extruder is extruding too much material, the material has to go somewhere.
• calibrate Z: same thing as above, if your z steps are off, you might not be moving up as much as you think you are. The extruder is calibrated to extrude a certain amount of material. If you end up extruding more material, it needs to go somewhere
• pressure advance: your nozzle may be oozing a little bit and causing your sides to become bowed