One of the quickest ways to tell the difference between a world-class player and a typical club player is to compare how far their feet are apart. Top players almost always have wider stances than average players. Just go to youtube and watch videos of the best players and the difference becomes obvious. The wider stance can be tricky to learn, and if you have knee problems, weak legs, or are overweight, it may not work for you. But for most players, the wider stance is a big advantage.
First, it allows you to keep your balance when moving and when making powerful shots. Table tennis is all about balance; if you are even slightly off balance it affects both your shot and (often more importantly) your recovery for the next shot. When you go for a big forehand, the wider stance keeps you balanced and stable.
Second, it increases your power. The wider stance allows you to put more weight into the shot since there’s a longer transfer period as weight goes from the back leg to the front leg. (This is especially true for forehand shots, but is also true for most backhand loops.) With a more narrow stance, you can only transfer the weight a short distance, and so you get less power. It’s like comparing a baseball pitcher with a long windup to one who tries to flick the ball with a short backswing – the longer windup will give more power.
Third, it allows you to move more quickly. This is both because your center of gravity is lower, allowing a quicker start, and because the wider stance gives more leverage.
The downside is it takes time to learn, and is also more tiring. But the payoff is worth it. So give it a try. It’ll seem different at first, and perhaps not comfortable. You might experiment by exaggerating it sometimes so that a more normal wide stance will seem more natural.
So consider widening your stance, and watch the improvement in your balance, power, and quickness.