Lidl used to sell 2Ah batteries in store only. You could get the bigger ones but they were only available online (https://www.kompernass.com). Finally, Lidl will have 4Ah batteries for sale in store from 19/08/2018. The battery considerably increases performance of the 20V family power tools (comparing to the 2Ah one).
I thought that higher Ah affects only usage time but apparently it affects power too due to the higher current available in the battery that some power tools (especially cordless grinder) can take advantage of. The more cells in parallel, the higher the current the battery can provide.