Here's another qupte from the DDSOpt guide:
Don't put in any mods that are already sufficiently well optimized or for other reasons don't benefit from texture optimization. Any mod in the STEP Mod Texture Properties table in this tab with No in column 2 should not be further optimized unless there is need to reduce texture sizes; even then it may be better to avoid additional optimization if possible.
I don't understand that sentence "even then it may be better to avoid additional optimization if possible." So how do I reduce texture size without additional optimizing?