This technical answer from Marcos (from the official ESET forum) answers this topic.
ESET's updates contain smart detections that are a few bytes in size, meaning that one detection that describes a particular malicious behavior or malware file characteristics can cover even thousands of variants in an ideal scenario. These are distributed either via pico updates or aggregated in engine updates together with other detections. Engine updates are optimized for distribution as differential updates.
Currently if only the engine was updated and a computer would be online 24x7, it would download engine updates 6 times a day, each about 11kB in size. If you missed 1 update, you'd download 15,5 kB. If 3 updates were missed, then a 21,4 kB engine would be downloaded, etc. ESET uses modular architecture, meaning that a particular feature can be often released (fixed, improved) via module updates instead of releasing a new version of the program. There are currently about 50 modules (archive module, HIPS module, fw module,...) which are updated more or less frequently depending on the needs.