One of the things about AV is it is not a product like a washing machine that you buy and you have it.
It rapidly goes out of date as new malware is released and so has to be kept up to date.
Consequently it's more of a service that you subscribe to.
One of the key things is how quickly the antivirus vendors respond to new threats - when something big and bad and nasty come out how quickly are you protected?
In general the bigger companies have more resource and better backend systems and respond to new threats faster.
Some of them acually do real-time lookups so if something suspicious is seen your computer checks with a central database - thatway they can give you protection within minutes of something being deemed bad
Of course you don't see all this and the bigger companies are not always very good abut explaining these benefits.
I know this stuff because I've spent the last 15 years working for one of the major vendors but I don't say which (so don't ask me) and I don't recommend individual software brands