There are many other things inside the HIV capsid besides RNA. The ssRNA is bound to the p7 nucleocapsid protein, the p6 late assembly protein, as well as integrase and reverse transcriptase, both of which are vital for infectivity. Also enclosed within the virion are Vif (Viral Infectivity Factor), protease, Nef (Negative Regulatory Factor), Tat (HIV Transactivator), and Vpr (Viral Protein R). Surrounding the capsid is a matrix formed by the p17 viral protein, and finally the lipid membrane envelope (taken from the host cell upon budding), which contains host cell proteins as well as the vital trimer of gp120/gp41 heterodimers, which bind to the target cell.
HIV (and many other viruses) depend on binding to certain cell surface receptors, and a larger total size increases the chances of binding to multiple receptors and potentially triggering receptor-mediated endocytosis, if that is part of the virus's entry strategy.
The naked λ phage binds to the lamB porin of single-celled prokaryotic E. coli by way of the phage's J protein and injects its DNA load into the bacterium, where host factors circularize and supercoil it, allowing for transcription to begin. While the phage's genomic material can be integrated into the host's chromosome, it is not required. Additionally, there is no nuclear barrier to cross.
From Wikimedia, numbers in parentheses are copies per virion.
On the other hand, HIV is a highly complex enveloped lentivirus that targets very specific cells in its multicellular eukaryotic host HIV requires reverse transcription and the assembly of a large pre-integration complex to transport the viral DNA into the nucleus and integrate it into the host's genome, all before it can even start producing new viral proteins.
Trying to compare the "packing efficiency" of HIV to the λ phage isn't just comparing apples to oranges, it's comparing apples to goats. The two viruses are completely different, with the only essential similarities being that they are viruses that infect cells.