You need all the RF circuitry as well as the silicon - crystal, the antenna matching circuit (the inductors make this hard to integrate with the silicon!) and possibly LNB/Power amp too if longer range is needed - look at the modules that use the nRF chip and you'll see its even larger than the Hope RF modules. In theory there can be substantially smaller RF modules (small xtal, minimal antenna-matching circuit), but I haven't seen any smaller than the 16mmx16mm of the HopeRF modules (not for a transceiver at least).
Designing an RF circuit from scratch using the nRF chip or similar would be doable if you have the right test equipment and understand RF design issues. An advantage of using stock modules is that they have been approved as complying with EMI/RFI regulations.