In most BJJ gyms you will find a number of people that strictly to stick to their own weight class when rolling and drilling. In general this a good idea but I do not think it is a strategy that needs to be followed religiously.
The primary reason smaller guys give for not rolling with bigger guys is that they want to avoid injury. It is true that bigger guys will generally possess more power and the risk of injury is higher (especially with big white belts). Sometimes it can take only one bad experience for somebody to decide that they will no longer train with anybody that is obviously outside of their weight. I think this is unfortunate when it happens as I find that this fear of getting hurt limits people’s ability to experience different styles and body types which ultimately help you to become a better grappler.
Different body type different moves learn
If you watch professional BJJ at the lighter weights you will immediately notice just how fast the smaller guys are when compared to the bigger guys. This speed allows them to pass and recover guard quickly. Smaller guys often prefer passing on their feet and techniques such as berimbolos and 50/50 guard are more common.
In comparison you will find that many bigger guys prefer smash passing styles as it is easier to pin an opponent as the strength to weight ratio is significantly lower resulting in less scrambling during the course of the round.
Rolling with both of these styles is a great way to see the other side of BJJ. If you are a small guy then yes, the chance of you being squashed is higher but at the same time it is an opportunity to experience a completely different game. If your half guard sweep works on the big guys too you then know for sure that your technique is solid!
Helps you get to know your teammates better
Rolling with the teammates that you do not usually roll with helps you to get to know them better. The few moments in between rolling and drilling are a great way to get to know someone a little better as you chit chat and talk about technique. If you practise with them regularly you are more likely to chat to them again when off the mats.
Knowing who your teammates is important as it helps to ensure a positive and friendly atmosphere as well as making a stronger overall team.
Less risk over time
As you get to know your different sized teammates you will also get to know their game. You will get to know the positions they like and will soon learn how to better defend them. If they are consistently doing something that you think is risky then you should be able to just say it to them so that you can roll freely without worrying. The worst thing you can do is to stay silent and avoid them every session.
If they are experienced they should understand the strength/size difference and will hopefully be less likely to crush your sternum with a knee but this one very much depends on the individual.