BD taking 800MB RAM? For a free AV with no extra features, that is just insane. It sure has come a long way from a light AV to this.
800 mb is not normal. It can reach that occasionally when it's performing scans, removing malware. It usually reserves 400-500 mb for itself and uses around 300 mb on average. They keep all of their signatures on ram.
Bitdefender free has the full engine as the paid versions. Signatures, behavior blocker, system wide web protection against malicious and phishing sites. So it has almost everything. The paid has extra features like Firewall, network blocker, ransomware remediation, banking protection and some other features which are not so necessary. Bitdefender is quite light on the CPU. Lighter than some other top products. Only the ram usage is high.