I read an interview with an ESET guy and he said that they 80% depend on heuristic scanning+DNAScan+ (I forgot lol) rather than their signature based detections. Thats why their daily update file size is merely 5-20 KB.
Their daily update file contain virus info of about only max 100 (http://www.eset.eu/podpora/aktualizacia-6295?lng=en) but this is 1000-4000 with avast.

I don’t know how they manage it but they manage it efficiently.

Also scan on demand scan is very slower than avast but ESET is lighter than avast otherwise loads quicker than avast. also when I suggested them Boot Scanning feature like avast they claim that their AV loads before system boots so they don’t need boot scan.

But also avast has 160+ million users so giving avast a very large virus database analysed perfectly. Also avast provide 2 update everyday but with ESET you hardly get any on Sunday or only 1 on Saturday so you can’t say they protect you from 0 day threats.

Thanks
Ash