Correct - RAM size does not define any QVD size limit per se.
But for a QVD to be created all the data one wishes to store in it has to held in RAM via the load script. People do on occasion run out of RAM to store the data and thus this in practice becomes the limiting factor size wise to creating a QVD.
There is though a limitation on the number of unique values that can be held in a single field, a touch over a billion I think but that may have increased as new versions of Qlik are released. But that is a limit within the QIX Engine's Symbol Tables / Bit Mapped Pointers and not QVD Files.