Because the engines are curbed. Nowadays these engine are curbed on this EU-wide microcar-class with Vmax 45 km/h. I don't know, what's the most possible ccm for them. For mopeds, having these plates, it's 50ccm

These tiny plates are used in Germany for
http://en.wikipedia.org/wiki/Vehicle_registration_plates_of_Germany#Insurance_pla- tes
http://de.wikipedia.org/wiki/Kfz-Kennzeichen_29#Versicherungskennzeichen

So the 1000ccm-engine is not in original condition/usage any more.

In the past there was the possibility for people, who made a moped-license before 1950, to drive any kind of vehicles up to 250ccm. But these cars and motorbikes had regular plates.
This regulation was one of the reasons, why all the 1950ies microcars in Germany were available with 250ccm.

The last original 250ccm-car was the Goggomobil, produced for that reason even until 1969. After its production was stopped, the assembling of the Goggo-engine continued! The production-"line" (a dozen of old workers and their handcraftsmanship) was moved from the former Glas- and now BMW-factory in Dingolfing to the former Borgward-factory in Bremen. In the edge of an otherwise empty plant-hall they made these 250ccm-engines especially for these old license-holders up to the 80ies. When Mercedes Benz needed the space for the production of the 190 [W201], these old Goggo-workers were sent into retirement.

Thes 250ccm-"big block"-engines were mounted mostly into the NSU Prinz 4 and Fiat/Jagst 600, later mainly into Fiat 500, 126 and Panda.

The downsizing to 250ccm was not only made by using Goggo-engines, also by dropping the engine-size, for example due blocking two pistons. This was usual at the curbed Fiats (127, Panda, Uno, Lancia Y10). In the 80ies mostly Fiats were used, some tiny Japanese cars as the Daihatsu Cuore only a few times.