Is there a difference between "la malplej grandan" and "la plej malgranda"
imagine you had six cups in six different sizes (from huge to tiny):
grandega, granda, grandeta, malgrandeta, malgranda, malgrandega
in this case "la malplej granda" (the least big) would be the grandeta one, and "la plej malgranda" (the most small) would be the malgrandega one. So by definition the meaning is not the same. If it is in a particular case depends on the context.
If I follow your line of reasoning correctly, then DuoLingo has gotten the English translation wrong. However I think that your reasoning is flawed, in which case the Esperanto should be "plej malganda", not "malplej granda".
Jes, "malplej granda" (least big) and "plej malgranda" (smallest) has the same meaning. Which one to choose is a matter of taste.
Deziri = To desire, want, wish (for) (to)
Voli = To will (use will-power), choose, determine, have a mind, intend.
I'd have to say no, but I personally think that Duo is using voli wrong anyhow.