The antonyms for the phrase 'most mini' would be large or huge. Most mini implies something that is the smallest or tiniest, but if we were to look for its opposite, we would go for the exact opposite in terms of size. Large or huge, indeed, convey the extreme end of the size spectrum, with massive and enormous being the most vigorous among them. If we use them in a sentence, we could say something like "the most mini fridge was not appropriate for the large items, so we had to buy a huge one." Therefore, one should choose the antonym carefully to match the intended meaning.