When it comes to square or round tables, typically a 42″ and 48″ will be able to sit the same amount of people just like a 54″ and 60″ can. When you sit 6 people at that kind of size, it is a very tight fit as well. Leg tables will be more constrained when it comes adding chairs due to the fact you lose your corners on each end.
Depending on the size of the legs, it may be possible to put 6 people around the table but this is rare and still may not be comfortable for everyone sitting.
This doesn’t increase the amount of people you can sit at your table. The one thing to keep in mind when you increase the width, it can make it more difficult passing dishes and condiments.