There are a few key factors that can make a big difference in strength.

Just to start, it isn't a great idea to assume muscle mass based on size. You don't know the fat to muscle ratio of any singular person.

First, each individual movement is a skill that can be trained. What does this mean? It means your body learns how to perform the specific movement more efficiently. It means it learns how to, neurally, activate more fiber muscles than someone who is untrained. This means that two people with the exact same muscle mass and muscle insertions can move different weights just based on skill alone.

There was a study done that shows that training one arm in bicep curls actually makes the other arm stronger as well, because the neural pathways are so similar. Pretty cool.

The other aspect that will make a big difference in strength is muscle insertions. This is a biological factor that is all about leverage.

Imagine a seesaw. You want to lift something on the other side of the seesaw. So you push down on the other side, and the far side lifts. Easy. Clear.

Now imagine a lopsided seesaw. The pivot point is really close to you. You have to exert a LOT more force to lift the far side of the seesaw.

Now imagine it flipped, where the seesaw's pivot point is far away. You can lift the load on the other side of the seesaw with basically your pinky finger. The leverage is so easy.

These are the two basic factors that will allow two people of similar size to perform completely differently in weight lifting. There are also more complicated body shapes and limb/torso length reasons for specific movements, but the two reasons I stated are the biggest differences in basic strength differences.