The first half of the year saw a shrinkage of turnover for Aldi of 4.1 percent according to figures the magazine took from the market research company GfK, which analyses the receipts of 30,000 consumers each month.
This drop is more than would be expected due to reduced prices, the researchers say.
Competitor Lidl saw its turnover shrink by 0.7 percent, a bigger drop than those experienced by traditional supermarkets.
One reason could be a reduction in demand for non-food items from the supermarkets concerned.
France is experiencing a similar trend, with a long spurt of discount supermarket growth now having hit the buffers. Between April and July the share of the market held by discounters dropped by 1.2 percent to 13.7 percent.