Some things are doable, others may tricky or impossible.
For example:
Changing the tooltip text to add some images: easy.
Doing math within format string: not working.
This is a PoC:
Lua Code:
SafeAddString(SI_ITEM_FORMAT_STR_DERIVED_STAT, "Adds |cffffff<<1>>|r <<2>>. |t25:25:esoui/art/mainmenu/menubar_inventory_down.dds|t\nAdds |cffffff40%|r of it. |t25:25:esoui/art/vendor/vendor_tabicon_fence_down.dds|t", 2)
Or you blank this line and add the information at the bottom. But beware. If you are showing something wrong, the players will blame you for everything.
And, by changing culture variant text, you have to do multi-language support.
The alignment will be a problem. You can not do text with dynamic length (the values) with in-line images and when say: "The images of the second line are aligned to the images of the first line". There is no markup to do this.
As Sasky said: You could change the text to give the hint about small and large pieces and when add a line at the bottom, which shows what small and large pieces are.
With "you" I mean a coder could do this for you