By default, HA Admission Control will "reserve" 50% of the resources for HA reasons (in two node cluster; in three node cluster it will be 33% by default). The idea is to be able to successfully assign resources during the failover process (in case of two hosts you should not allocate more than 50% of total available resources to satisfy the possible failover requirements).
The exact configuration depends on your workloads and protection requirements; in your situation, it might be more useful to disable HA Admission Control at all because the "costs" of those reserved resources will be quite high.
There is no direct correlation between HA / DRS and Admission control. You can fine-tune HA / DRS with Admission Control disabled, but keep in mind that during the unplanned failure of the host your environment might not have enough resources to run all your VMs without impacting the performance