Adjective/noun to describe a small but extremely useful thing
Solution 1:
Maybe "specialized" or "niche product". I also like Dror's suggestion of "peppy".
But frankly, I think you're looking for too specific a word. I'd suggest you use a phrase or sentence to explain what you mean. It's not always possible to find a single word that expresses any idea you can possibly think of.
Solution 2:
Packed? Packed emphasizes its high performance to size ratio, regardless of the actual size though.
Similarly, packs a punch.