The capacity information in the Kodak datasheet linked to by Thomas indicates to me that Kodak recommends that you use at least 250 ml of stock D-76 per 135-36 or 120 roll.
So if you are using D-76 1 + 3, they are essentially recommending you use at least one litre of working solution per roll.
They don't, of course, make any recommendations for 1 + 3, but you can extrapolate the stock and 1 + 1 data.
They tend to be quite conservative in their capacity recommendations, but if they are recommending 1 litre, and you are using only 400 ml, you have probably gone outside any margin for error.
If you use smaller amounts, you are likely exhausting the developer in the highlight areas, and should expect lower than normal contrast.