How can uTorrent be multi-platform while keeping such a small binary size?

Question

Nowdays it turned into a bloated client that works in Mac, Linux, and Windows, while still keeping the whole program in a single binary file that weighs only 876 kb, while competitors that use gui libraries weigh around 6 mb.

Do they create a separate native gui for each platform instead of using a multiplatform gui library? What's the best approach to achieve such a thing?
Thanks.