Scanning files manually

When I scan a file from a certain size manually, by right-clicking and then selecting “Comodo Antivirus”, it just doesn’t seem to scan certain files. Changing the “Do not scan files larger than…”-setting in Scanner Settings doesn’t help either. The “Total Objects Scanned”-message remains 0. When I select a much smaller file, like 2 MB or smaller, it does seem to scan it (at least gives a message that one object was scanned).

Another thing, which worries me a little: I used Avast before, and that always seemed to take some time to scan something. Comodo Antivirus is always finished under a second, thereby giving me the idea that it actually does very little. It just doesn’t make me feel safe.

So, my questions are:
[1] How can I be sure that the antivirus scans everything I manually select by doing the right-click scan, nomatter the file size?
[2] Why is the antivirus so fast?