According to eheims website:
Now the pre-cleaned water falls to the bottom of the canister and starts to flow through 3 filter baskets which hold 4 l respectively (filter volume: 12 l + 1.5 l pre-filter = 13.5 l). Thanks to these easy-to-use filter baskets, partial/complete cleaning or partial/complete replacement of the filter media is much easier. The professionel 3 complete filter media set 2520800 – all filter media for your EHEIM filter in one set – is available as an accessory.
Other web searches also bring up a figure of 13.5 litre capasity for the 2080.
Couldn't find the figures for the 2028 on eheims website but other searches say its 7.3 litres.