I had a PC playing a medium-sized hill giant, and we decided that at 5th level, he could grow to Large size instead of getting Extra Attack. So, rather than making two attacks with a maul at 2d6+Str, he would get one attack at 4d6+Str. Seems balanced, right? If you do the math, it's actually a downgrade. Except that this player figured out how to get extra attacks, via frenzied rage, haste, and Great Weapon Master.
In order to do 4d6 per attack he's presumably not using a shield, so his AC won't be very good, while he occupies a 4x4 area, making him a huge magnet for enemy attacks.
IME Large PCs are often a liability to their own side, especially in a confined battle space, as they block access routes - it takes 25' of movement just to get past one through the area they occupy.