Frequency refers to how often you are targeting your muscle groups over a period of time
e.g. in a week
Volume refers to the number of sets done over a period of time.
Based on this
study, it clearly shows that when volume is the same, training each muscle group at least 2 - 3x a week results in greater muscle growth as compared to just targeting each muscle group just 1x a week in a Bro Split.
Higher training frequencies would also allow better protein synthesis response throughout the week but also allow you to perform
higher quality sets.For example if you were perform a Chest Day like on a Bro Split with 4 sets of 4 different exercises, you would find yourself starting to fatigue after your first exercise and your performance will undoubtedly decrease by the time you reach the last exercise.
On the other hand if you were to split your chest exercises into 2 different days with 2 sets of 2 different exercises each, you can better optimised your training with higher quality reps and sets since you are not doing as much volume all at once.
Hence as you can see training with Bro Split with low frequency might not be as optimised for muscle growth as compared to any of the 3 higher frequency workout splits mentioned earlier