I basically want to know if DDOSing using a single pc with 20 threads on HOIC or LOIC be different based on sites? There is a project website I made which goes down in a minute after the DDOS starts. (www.aroosani.net) But there are some other medium-size ones that don't even slow down. So how do I know if a site can handle less or more bandwidth?
Have you checked whether this is actually bandwidth related? Is the bandwidth actually max'd out? Or is it the web site itself that cannot keep up with the volume of incoming requests?
Generally, the last is more likely and would often be why you see the issue on one site and not another. The number of requests a site can handle is heavily dependent on the server technology being used (both the web server and any scripting such as PHP or Node.JS), page sizes and complexity, memory utilisation and availability and so on.
You need to provide some stats from the sites if you need specific help. You also need to share what technology you are using. Both software and hardware.
As a final note, the use of a CDN can massively help protect against DDOS. Services such as Cloudflare have a number of features that will both protect against and mitigate DDOS attacks on your site.
Usually, apache webservers hosted on low-resources like VPS are more vulnerable to DOS attacks due to his process-based system and his slow http "vulnerability".