If an app allocates it and ever uses it and refuses to give it up unless killed that can be considered wasted. It's called a memory leak and they can be really bad, especially when they consume a lot of memory, as that memory might as well be empty but is being held hostage by other apps.

If they released RAM then whatever amount they were using wouldn't be wasted and if more is needed they'd simply release it to free up resources. That hasn't been happening though, and most modern Browsers are notorious for consuming massive quantities without releasing it back to the pool.

In that case with the presence of Memory leaks being considered, and the fact that they continue to not be fixed, the acceptable amount of RAM a browser should be using (should even have access to) is the minimum necessary to run smoothly. From my testing with Firefox that seems to be 8GB. 4GB caused many websites to struggle. Such an arrangement ensures that even if a Browser begins eating RAM it won't eat up all the RAM and cause issues, worst that'll happen is that it itself will crash from eating all the 8GB it was allowed to access.