Besides muscle mass, there are many other factors that affect muscle strength. You might have seen people who look "weak", but when they start picking up heavy things you get surprised at how they are able to achieve such feats of strength considering the size of their muscles and body stature. And there are opposite examples as well - bodybuilders (people whose goal is to maximize muscle mass) who get overpowered by much smaller power lifters (people whose goal is to maximize strength) for example.
Here are some videos showing amazing feats of strength by people who might not look like being capable of moving such heavy weights:
Cheryl Anderson (bodyweight 97 ibs) lifting 315 ib
Weightlifter Chen Wei-Ling (bodyweight 100 ibs) squatting 440ibs
Jennifer Thompson bench pressing 315ibs
17 Year-old girl outpeforming many professional athletes.
Man bench pressing 410 ibs at 154ibs bodyweight
So what makes this people so strong? Here are 7 main factors that determine muscle strength: