The biology that makes 'accidental bulk' unlikely
Building significant muscle size requires a sustained calorie surplus, a lot of training volume, and time — usually years. Women also naturally carry roughly 10-20 times less testosterone than men, the hormone most responsible for muscle size gains, which makes rapid, dramatic size increases from a couple of gym sessions a week essentially impossible.
What actually happens when women lift heavy
Increased strength, better bone density, a more defined shape (from muscle under existing fat), and a higher resting metabolism. The 'bulky' look seen in competitive bodybuilders comes from years of deliberate training, eating, and sometimes other interventions — not from a few strength sessions a week.
If the look does start to change more than you want
You have full control — reducing training volume or calorie intake slows or reverses muscle growth just as it would build it. Nothing that happens from lifting is permanent or accidental.
The takeaway
The fear is almost always disconnected from the actual physiology. Lifting heavier is one of the most effective things you can do for strength, body composition, and long-term health — the 'bulk' most people fear takes deliberate, sustained effort to achieve.