DNS RR records are very small, somewhere in the neighborhood of 16 bytes. Domain records are also very small, they are around 56 bytes. If you load a great deal of records into the server you might not see the DNS process Private Bytes counter jump because the DNS server allocates the memory in blocks.
The way DNS uses the cache is a different story. Virtual (pageable) memory is allocated for cached entries as needed (remember, as a DNS server does a resolution, it caches a copy of the data for that data's TTL). If the server is starved for memory and more is needed, then this memory will come from the page file (which is usually very slow).