Firstly, are you certain the first is six litres, have you not been confused by the arcane markings on the largest containers I am aware of: 3+ litres or 6 pints ?
The milk itself is unlikely to do any harm, some would suggest the contrary. As woofgang says, sugar is more of a cause for concern but your information regarding the diet (sum total) is far more extraordinary and seems destined to prescribe some sort of health disaster.
For comparison, I know someone who has never drunk tea of coffee and next to no alcohol but instead milk is the main source of pure liquid. He is not overweight and very healthy and active compared to his peers (I know of nobody among them who is healthier/fitter), takes no medicines of any kind and has a cold once every 3-4 years if that, never has flu. He will be 70 on his next birthday. Oh, and he is hopelessly addicted to good chocolate and eats substantial quantities of it - but, possibly crucially, his diet is balanced with well above average amounts/proportions of fruit and vegetables.