check that your wall thickness is a sensible multiplier of your extrusion width and layer height.

ie: if you want 1.5mm wall thickness you need to be using an extrusion width that fits within that. so 2 (inner and outer = 4 layers) layers at 0.4 is 1.6 which enlarges the print slightly.
So better option would be one wall layer and 100% infill.

Likewise if you print something 0.8mm thick (like my sling pouches) then you need to use 0.2mm layer for 4 layers. Or use several processes to get the right height in simplify3d.

None of it's complicated - but you do need to take basic aritmetic into account when you design something for printing.

Also abs tends to shrink about 1% of the overall size, so I usually enlarge the print by 1% on the slicer bed for abs and that usually sorts it.
Pla usually prints the size I wanted - unless I got the maths wrong :-)

so just check the numbers :-)