They don't have the same frame, they have the same FLOORPAN, and as we all know it is now coming practice for the VAG group to share floorpans amongst their cars.
Its the same as the MK4 golf and Audi TT having the same floorpan, the shape of the actual car is completely different.
The only VW I know that looks externally like any ibiza is the MK3 polo saloon/caddy van.