Men have exponentially more testosterone in their bodies than women and testosterone drives the development of male sex characteristics.

For women that want to undergo sex reassignment, the first line of therapy is testosterone, because it initiates the major changes that will turn a woman into a man.

Because women don't have the hormone levels to sustain any serious amount of muscle, especially upper body muscle (delts, traps, biceps, pecs, in particular, don't really grow much for natural women), it looks highly abnormal on a woman and is going to throw off most people. The development of those upper body muscle groups is a very distinct and clear indicator of maleness.

Taking testosterone, honestly, causing long lasting changes in women that don't happen in men and can really fuck them up. It's not to be taken lightly like most guys that can jump on a cycle, because, as I said, it's basically HRT. That's why most women never get into BBing and just stick to physique where they can get away without taking it.