The X-dimension setting of a barcode specifies the width of the barcode; this, in fact, adjusts the number of dots a printer uses for the width of the narrowest element of the barcode. As a result, all other bars of greater width grow in ratio to the narrowest bar as you expand the barcode's width, which is what causes the "jump."
Printer resolution determines available sizes
The size of the dot and the X-dimension are determined by the selected printer's resolution. A dot represents the size of the smallest mark the printer can print. Each step up of the X-dimension increases the narrowest element by a single dot. The printer resolution is important because it's impossible to increase the narrowest elements by fractions of a dot; it can only increase by a full extra dot. Of course all wider barcode elements grow in ratio to this.
Suppose the narrowest element is 1 dot wide and the widest element is 10, when we increase the X-dimension by 1 so that the narrow element is 2, the widest element will now be 20. This accounts for the "jumping" in the barcode's width when you increase the X-dimension. The higher the resolution of the printer, the smaller these increments will be, because the size of a single dot will be smaller.
Using images of barcodes
We do not advise that you try working around this physical limitation of the printer by exporting the barcode object as a graphic and then importing it back into your design as a picture object. Although a picture object can be scaled to any dimension desired, it will result in a poor quality barcode and may be unreadable by a barcode scanner.
Targeting a specific X-dimension
BarTender allows you to specify a target x-dimension of your barcode, which will tell BarTender to always render your barcode as close to the target x-dimension as possible. This feature can be found in the properties of the barcode on the Symbology and Size page. Click on the Wizard button to the right of X Dimension.