How do I make different weft thicknesses all come out the same width on the final product if the warp is all the same thickness? Fulling helps it but it still looks odd.
This is always a tricky situation. Fat wefts will always push your warp out and thin wefts will always pull your warp in. So you can do 2 things. You can build a pattern into the design where you use those tendencies as a design element, i.e. make regular stripes of the two yarns so it looks like you did it on purpose.
The second thing you could do is to purposefully draw in more when you are using your fat yarn. Draw it into the width of your fine weft area. Or when you throw a pick of your fine weft you need to leave that yarn at a higher angle so that more weft can get into the shed, this will control the draw-in on the fine weft. It really all comes down to selvedge control, but knowing that fat wefts and thin wefts will always cause these concerns you can either avoid the situation or use it as a design element.
One more thing………if you want, you could wind several ends of the fine to make a thicker weft and that would reduce the problem, but if you are after the thick/thin effect, that won’t work.