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.
I do not agree,in my humble opinion, as seen above. why ? the least big (e.g of the cats living on the earth) is still one in the group of the biggest (e.g mammal species) and the most small or the smallest (of mice or whatever) is part of the group of the smallest (e.g mammals).
quod erat demonstrandum
I am happy to be corrected.