In my experience there is absolutely no discernable bandwidth compromise from the Breakout Rooms, but the technical answer and the Best Practices answer would be equally relevant here
Imagine Breakout Rooms as ‘concurrent meetings’. If I have 20 people and took them into 4 breakout rooms with 5 people in each then the bandwidth is identical to the original 20 people in one room
The change to consider is that by definition, people in a Breakout Room are going to be far more interactive as ‘Presenters’ than when they were ‘Participants’
In the example above, if each Breakout Room had a person on camera then that equates to 4 camera feeds rather than the one feed you might typically see in a regular room. If the 4 people are also screen-sharing then you have 4 screen-sharing feeds as well
So again, if you consider a Breakout Room setup to be the same as running concurrent Meeting Rooms then you will understand the bandwidth usage and you might consider putting in place some best practices around camera use etc. that respects ho you might protect your bandwidth