Also, if you patch a tube that's meaningfully too small, your patch may have trouble staying on because the contact patch/glue will be extra stretched, increasing chance of separate between patch and tube.Tubes have a range of compatible tire widths printed on the box and often on the tubes themselves. If your tire fall is that range, you're golden. If it's outside of that range a small amount, you're OK too. A tube that's too big can get folded or creased in the tire which can cause a flat due to abrasion with itself. A tube that's too small will expand to fill the tire, but it will be stretched which can make it easier to puncture.