The discounting curves are generally inferred from zero coupon bonds, especially for short rates, where such zero coupon bonds exist. Given the recent governmental negative interest rate bonds, this implies that the discounting curve has to be negative, at least for short rates.
Because the observed increase in magnitude of negative interest rates can be justified by an increase in the perceived credit risk, the momentum profitability could drive the future investments. Within such a model, historical data analysis shows that, for high risk, momentum profitability decreases with size, so volume might be capped, depending on the specific credit rating of the considered portfolio/counterparty.
Observed is that the amount which can be borrowed by an entity is limited (even at LIBOR rate, one can borrow in a "reasonable market size", which is carefully monitored). Depositing now might be treated as collateral to borrowing in the future, therefore involved in calculations with an associated haircut. This might lead to a maximum depositing amount, with the magnitude of negative interest rates increment depending on the left size of the deposit-able amount.
Should the magnitude of the negative interest rates depend on the already deposited volume, in addition to the credit quality of the two sides, and which relationship might put all the three together?