The "compact dictionary" representation you're talking about not always using 64-bit numbers, but rather using log(n)-bit values when you have n entries (i.e. using 8-bit values when there's only <= 128 entries). The paper linked here talks about pointers which uses less than log(n)-bits for n entries.
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The "compact dictionary" representation you're talking about not always using 64-bit numbers, but rather using log(n)-bit values when you have n entries (i.e. using 8-bit values when there's only <= 128 entries). The paper linked here talks about pointers which uses less than log(n)-bits for n entries.